Zirconium and hafnium complexes containing N-alkyl-substituted amine biphenolate ligands: Unexpected ligand degradation and divergent complex constitutions governed by N-alkyls

Article abstract of DOI:10.1021/ic400891b

The reactivity and thermal stability of zirconium and hafnium complexes containing the N-alkyl-substituted amine biphenolate ligands of the type [RN(CH₂-2-O-3,5-C₆H₂(tBu)₂) ₂]^2- ([R-ONO]^2-; R = tBu (1a), iPr (1b), or nPr (1c)) were investigated. The reactions of either [1a]M(OiPr)₂ (M = Zr or Hf) with equimolar H₂[1a] or M(OiPr)₄(HOiPr) (M = Zr or Hf) with 2 equiv of H₂[1a] at 25 C in diethyl ether or 80 C in toluene afford moderate yields of colorless crystals of M[1a](OiPr)(iPrOCH ₂-2-O-3,5-C₆H₂(tBu)₂) (M = Zr (4a) or Hf (5a)). Controlled experiments revealed that the production of 4a and 5a proceeds via unexpected thermal degradation of H₂[1a] that produces a highly reactive, transient ortho-quinone methide intermediate. Similar reactions employing H₂[1b] and H₂[1c], however, led to the formation of homoleptic bis-ligand complexes Zr[1b]₂ (8b) and M[1c]₂ (M = Zr (8c) or Hf (9c)) as colorless crystals. Decisive factors governing these divergent reaction pathways and complex constitutions are discussed. The X-ray structures of 4a, 5a, 8b, 8c, and 9c are presented.

Full text of DOI:10.1021/ic400891b

Article
pubs.acs.org/IC
Zirconium and Hafnium Complexes Containing N‑Alkyl-Substituted
Amine Biphenolate Ligands: Unexpected Ligand Degradation and
Divergent Complex Constitutions Governed by N‑Alkyls
Lan-Chang Liang,* Chia-Cheng Chien, Ming-Tsz Chen, and Sheng-Ta Lin
Department of Chemistry and Center for Nanoscience & Nanotechnology, National Sun Yat-sen University, Kaohsiung 80424,
Taiwan
*
S Supporting Information
ABSTRACT: The reactivity and thermal stability of zirconium and
hafnium complexes containing the N-alkyl-substituted amine
2
−
biphenolate ligands of the type [RN(CH -2-O-3,5-C H (tBu) ) ]
(
2
6
2
2 2
2
−
[R-ONO] ; R = tBu (1a), iPr (1b), or nPr (1c)) were
investigated. The reactions of either [1a]M(OiPr) (M = Zr or
2
Hf) with equimolar H [1a] or M(OiPr) (HOiPr) (M = Zr or Hf)
2
4
with 2 equiv of H [1a] at 25 °C in diethyl ether or 80 °C in toluene
2
afford moderate yields of colorless crystals of M[1a](OiPr)-
(
iPrOCH -2-O-3,5-C H (tBu) ) (M = Zr (4a) or Hf (5a)).
2 6 2 2
Controlled experiments revealed that the production of 4a and 5a
proceeds via unexpected thermal degradation of H [1a] that
2
produces a highly reactive, transient ortho-quinone methide
intermediate. Similar reactions employing H [1b] and H [1c],
2
2
however, led to the formation of homoleptic bis-ligand complexes Zr[1b] (8b) and M[1c] (M = Zr (8c) or Hf (9c)) as
2
2
colorless crystals. Decisive factors governing these divergent reaction pathways and complex constitutions are discussed. The X-
ray structures of 4a, 5a, 8b, 8c, and 9c are presented.
INTRODUCTION
■
There has been considerable interest in the past decades in
amine phenolate complexes of main group and transition
metals because of their versatile structural and reaction
chemistry. Of particular note are their applications in the
construction of tailor-made molecular architectures for specific
1
−3
4−12
materials
or as catalysts for polymerization of α-olefins
13−22
or heterocyclic molecules.
The amine phenolate ligands
ancillary ligands and usually do not participate in o-QM
formation.
We are exploring reaction and structural coordination
are routinely synthesized by traditional Mannich reactions
employing appropriate phenols and amines in the presence of
2
3,24
formaldehyde.
With the incorporation of variable numbers
chemistry employing complexes of chelating phenolate
of N- and O-donors and a wide variety of peripheral
substituents on the phenol rings, these phenolic Mannich
bases have become conceivably ubiquitous ancillary ligands in
3
2−39
ligands.
In particular, a series of amine biphenolate ligands
2−
2−
of the type [RN(CH -2-O-3,5-C H (tBu) ) ] ([R-ONO] ;
2
6
2
2 2
5
,25
R = tBu (1a), iPr (1b), or nPr (1c)) featuring an N-bound
tertiary, secondary, or primary alkyl substituent, respectively,
has been employed to prepare complexes of group 1, 4, and 13
metals; their corresponding reactivity was explored there-
coordination chemistry.
ortho-Quinone methides (o-QMs) are transient but versatile
intermediates that have recently been demonstrated to be
26−28
competent synthons for natural product synthesis.
With
4
0−43
after.
In the course of the preparation of mono-ONO-
inherently highly reactive characteristics, o-QMs are susceptible
to cycloaddition, typically termed hetero-Diels−Alder reactions,
2−
ligated zirconium and hafnium complexes of [1] (Scheme
1), we observed in some cases inevitable generation of either
43
29
with vinyl substrates. Intrinsically, o-QMs generally exist as di-
3
0,31
bis-ligated derivatives or unidentified side products, depending
or trimers (eq 1).
Synthetically, o-QMs may be generated
on synthetic strategies and use of H [1]. To clarify, we
2
by thermal or photochemical processes either from their
scrutinized these reactions further and found that the amine
oligomeric precursors or from phenol derivatives bearing ortho-
27
substituted vinyl, hydroxymethyl, (pseudo)halomethyl, etc. In
contrast, aminophenols such as those that are widely used in
exploratory coordination chemistry typically act as “innocent”
Received: April 10, 2013
©
XXXX American Chemical Society
A
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Article
Scheme 1
Scheme 2
Figure 1. Molecular structures of 4a (left) and 5a (right) with thermal ellipsoids drawn at the 35% probability level. All methyl groups in the aryl tert-
butyl and isopropoxide are omitted for clarity.
biphenol H [1] may degrade to form an o-QM derivative,
reaction parameters, including reaction solvent, temperature,
time, addition sequence, and stoichiometry of starting materials,
we found that the same side products could become
2
depending on the N-alkyl. Though o-QMs were first postulated
4
4
45
in 1907 and spectroscopically observed in 1971, it was not
until 1998 that their absolute structures were elucidated by
1
predominant in the mixture produced as indicated by H
4
Amouri et al. as an η ligand in pentamethylcyclopentadienyl
NMR spectra when 2-fold H [1a] was reacted with Zr-
2
4
6
iridium complexes. In this Article, we present evidence of the
presence of o-QM in group 4 ONO chemistry as a consequence
of amine biphenol degradation and its participation in reactions
with reactive metal alkoxides. Parallel studies were also carried
out with distinct N-alkyl-substituted compounds in order to
probe decisive factors for how and why this degradation occurs.
These results are intriguing and informative, particularly in view
of the current interest in reaction chemistry involving Mannich
base-derived metal complexes.
(
OiPr) (HOiPr) either in diethyl ether at 25 °C or in toluene
4
at 80 °C (Scheme 2). Note that this reaction proceeds much
faster at elevated temperatures, though the ultimate thermody-
namic products are produced in essentially identical ratios. The
same results were also observed from the reaction of H [1a]
2
with 1 equiv of isolated 2a, indicating unambiguously the
participation of a second amine biphenol ligand per zirconium
in the generation of this unexpected complex. Similar
phenomena were also found for reactions involving H [1a],
2
Hf(OiPr) (HOiPr), and 3a.
4
RESULTS AND DISCUSSION
Complexes 4a and 5a were both isolated as colorless
crystalline solids in moderate yield with the strategies illustrated
in Scheme 2. Colorless crystals of 4a and 5a suitable for X-ray
diffraction analysis were grown by layering diethyl ether and
acetonitrile on top of a concentrated pentane solution at −35
°C. As depicted in Figure 1, these complexes are six-coordinate,
pseudo-octahedral species containing one intact meridional
[1a]2 ligand, one isopropoxide, and a novel phenolate ligand
■
It has been shown that the synthesis of complexes 2a−c and
43
3
a−c is not trivial. Though [1a]Zr(OiPr) (2a) could be
2
isolated from the reaction of Zr(OiPr) (HOiPr) with 1 equiv of
4
H [1a] in diethyl ether, some unidentified side products (ca.
2
1
1
0% as judged by H NMR) might concomitantly be
4
3
produced. Attempts to selectively isolate or characterize
these side products were not successful. After close scrutiny of
−
B
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Inorganic Chemistry
Article
tethered with an isopropyl ethereal donor. Complexes 4a and
(Table 2). While catalytically active, 4a reacts at a much slower
rate than 2a, producing poly(ε-caprolactone) (PCL) with
reasonably narrow molecular weight distributions. The slower
reaction rate of the former is ascribed to its more sterically
encumbered coordination sphere and a presumably less
electrophilic metal center when compared to the latter. Though
relatively slow, the reactivity of 4a in ROP catalysis is
informative as it suggests that complexes or intermediates
derived from thermal degradation may also function as
catalytically active species. This result is particularly relevant
to systems employing in situ prepared catalysts wherein ligands
5
a are isostructural to each other, though one is the enantiomer
of the other in the established absolute configuration. In
accordance with the electron-releasing abilities of substituents
at anionic O-donors, the M−O distances for aryloxides are
longer than those for alkoxides (Table 1). Without the anionic
charge, the isopropyl ethereal donor O(4) lies much farther
from the metallic center than other O-donors.
Table 1. Selected Bond Distances (Å) and Angles (deg) for
4
a and 5a
1
or additives are used in (slightly) excess amounts. The H
4
a
5a
NMR analysis of PCL (see Supporting Information) prepared
by catalytic 4a confirmed the presence of an isopropyl ester end
group instead of the incorporation of the ethereal phenolate or
ONO, suggesting that the initiation of this ROP catalysis
M−O(1)
M−O(2)
M−O(3)
M−O(4)
M−O(5)
M−N(1)
2.013(2)
2.015(2)
1.985(2)
2.469(2)
1.923(2)
2.547(2)
154.78(9)
93.69(9)
83.16(8)
98.76(10)
78.68(8)
101.63(9)
81.84(8)
98.12(9)
79.44(8)
74.45(8)
100.14(10)
156.87(9)
174.41(8)
82.93(8)
102.58(9)
2.0063(17)
2.0094(17)
1.9857(18)
2.4122(18)
1.9132(19)
2.492(2)
43
selectively involves the zirconium-bound isopropoxide ligand
47
rather than chelating phenolates.
Mechanistically, the formation of 4a and 5a may involve
O(1)−M−O(2)
O(1)−M−O(3)
O(1)−M−O(4)
O(1)−M−O(5)
O(1)−M−N(1)
O(2)−M−O(3)
O(2)−M−O(4)
O(2)−M−O(5)
O(2)−M−N(1)
O(3)−M−O(4)
O(3)−M−O(5)
O(3)−M−N(1)
O(4)−M−O(5)
O(4)−M−N(1)
O(5)−M−N(1)
156.10(8)
102.06(7)
80.94(7)
either 2a/3a or H [1a] decomposition in view of the
2
incorporation of the ethereal phenolate ligand in these
complexes. Controlled experiments showed that both 2a and
101.59(8)
80.70(6)
3a are thermally stable (40 mM in toluene-d at 80 °C for 24
8
92.28(7)
h), thus eliminating the possibility of 4a/5a evolution from 2a/
3a decomposition under the conditions employed. The ligand
84.39(7)
98.84(8)
precursor H [1a], however, decomposes gradually in solutions
2
79.33(7)
even at ambient temperature. The disappearance rate of H [1a]
2
−2
75.35(7)
follows second-order kinetics (rate constant k = 2.8(5) × 10
−1
−1
98.24(8)
M
min at 80 °C; see Supporting Information), suggesting a
159.42(8)
172.99(7)
85.09(6)
bimolecular degradation process. Analyses on the thermal
decomposition products of H [1a] (49 mM in toluene-d at 80
2
8
1
°
C) by H NMR and mass spectrometry revealed the presence
101.59(8)
of 2,4-di-tert-butyl-6-(tert-butylaminomethyl)phenol (6a) ac-
companied by di- and trimeric di-tert-butyl-substituted o-QM
1
([7] , x = 2 or 3; see Supporting Information) as depicted in
x
The solution structures of 4a and 5a were elucidated by H,
1
3
Scheme 3. The diagnostic signals of 6a and oligomeric 7 were
also found in reactions used to prepare 4a and 5a (Scheme 2)
C, COSY, and HMQC NMR experiments. Both molecules
are Cs symmetric on the NMR time scale. The averaged mirror
plane coincides approximately with the N−M−O(iPr) plane
that contains roughly the ethereal phenolate ring and bisects
1
by H NMR and mass spectrometry. Accordingly, we reason
that the transient monomeric 7 was produced upon thermolysis
2
−
of H [1a]; subsequent reactions of 7 with 2a or 3a took place
2
[
1a] . Diagnostically, the aromatic protons resonate as four
1
via insertion of the reactive methylene group of 7 into the M−
OiPr bond of 2a or 3a, thus forming the ethereal phenolate
ligand in 4a or 5a, respectively. This result is interesting in view
of the formally nucleophilic substitution of a benzyl amine with
an alkoxide. Consistent with these results, thermolysis of
well-resolved signals with an integral ratio of 2:2:1:1 in the H
NMR spectra. The N-bound benzylic methylene protons are
diastereotopic as evidenced by signals corresponding to an AB
spin system, indicating the coordination of the amine nitrogen
donor to these group 4 metals.
H [1a] in the presence of 10 equiv of norbornadiene in toluene
It has been demonstrated that 2a is a competent catalyst for
ring-opening polymerization (ROP) of ε-caprolactone and that
the identity of N-alkyls in 2 and 3 has a dramatic effect on both
coordination chemistry and ROP catalysis. The reactivity of
a was examined in comparison with its synthetic precursor
2
at 110 °C generates, via hetero-Diels−Alder reactions involving
7, the anticipated mono- and dicycloaddition products in 12 h
as evidenced by EI-MS spectroscopy (see Supporting
Information). The involvement of transient o-QM was also
43
4
a
Table 2. Catalytic ROP of ε-CL
b
M (calcd, kg mol−1)c
M (exptl, kg mol−1)d
corrected M (exptl, kg mol−1)e
d
entry
cat.
time (h)
conv (%)
PDI
n
n
n
f
1
2a
2a
4a
0.5
1
69
>99
96
7.9
11.4
11.0
6.3
10.0
15.7
3.5
5.6
8.8
1.13
1.12
1.31
f
2
3
72
a
b
1
c
Unless otherwise noted, [cat.] = 1.0 mM and [ε-CL] = 100 mM in 10 mL of toluene, 25 °C. Determined by H NMR analysis. Calculated from
0
0
d
[
(fw of ε-CL) × ([ε-CL] /[cat.] ) × conversion] + (fw of iPrOH), assuming one propagating chain per zirconium atom. Measured by gel
0 0
e 48,49 f
permeation chromatography in tetrahydrofuran, calibrated with polystyrene standards. Multiplied by a factor of 0.56.
Data selected from
reference 43.
C
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Scheme 3
Scheme 4
5
0
postulated in the formation of a yttrium complex containing
ably occurs upon electron release from O to N via the benzyl
moiety involving dearomatization, leading to C−N bond
cleavage and 6a and 7 generation. The possibilities involving
radical processes were also examined, but these postulations
were ruled out as identical results were obtained when reactions
depicted in Scheme 2 were conducted in the presence of 2,6-di-
tert-butyl-4-methylphenol, a common radical scavenger. Anoth-
er assumption concerns the possible involvement of 2,4-di-tert-
butyl-6-(hydroxymethyl)phenol, as salicyl alcohols are known
to react with metal alkoxides to generate ethereal phenolate
an N-heterocyclic carbene-functionalized phenolate ligand.
Though conformationally similar, H [1a] distinguishes itself
from H [1b] and H [1c] as the latter two are both thermally
stable as evidenced by H NMR studies (40 mM in toluene-d8
at 80 °C for 24 h). The thermal stability of these amine
biphenol compounds thus depends clearly on the identity of
their N-alkyl substituents. We reason that the tert-butyl-
substituted amine in H [1a] is substantially more nucleophilic
than the isopropyl- and n-propyl-substituted amine in H [1b]
2
2
2
1
2
2
51
and H [1c], respectively, thereby facilitating deprotonation of
the phenolic OH, presumably in a reversible manner. Note that
complexes. Compound 2,4-di-tert-butyl-6-(hydroxymethyl)-
phenol could in principle be produced accompanied by 6a by
2
X-ray studies of H [1a−c] revealed intra- and intermolecular
hydrolysis of H [1a]. Controlled experiments, however,
2
2
52
hydrogen bonding of hydroxyl with the amine nitrogen atom
revealed that 2,4-di-tert-butyl-6-(hydroxymethyl)phenol was
40
1
and cocrystallized acetonitrile, respectively. The driving force
not produced ( H NMR evidence) from the reactions of
for H [1a] rather than H [1b] or H [1c] degradation is thus
putatively ascribed to the somewhat stabilized protonated
H [1a] with water (25 °C in diethyl ether or 80 °C in toluene),
nor was 4a/5a generated ( H NMR evidence) from the
2
2
2
2
1
ammonium nitrogen in the proposed transition state A
reactions of 2a/3a with 2,4-di-tert-butyl-6-(hydroxymethyl)-
phenol. Consequently, the participation of 2,4-di-tert-butyl-6-
(hydroxymethyl)phenol in the formation of 4a/5a was also
(Scheme 3) because of the presence of the tert-butyl substituent
whose electron-donating nature is stronger than that of
isopropyl and n-propyl substituents. The degradation presum-
ruled out. Conceptually, the degradation of H [1a] may occur
2
D
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Inorganic Chemistry
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Figure 2. Molecular structures of 8b (left), 8c (middle), and 9c (right) with thermal ellipsoids drawn at the 35% probability level. All methyl groups
in the aryl tert-butyl are omitted for clarity.
upon nitrogen coordination to the metal center of 2a or 3a, but
this hypothesis is perhaps unfavorable because the steric size of
H [1a] is larger than that of H [1b] or H [1c], with which
ligand degradation does not take place in reactions similar to
that illustrated in Scheme 2 (vide infra).
Table 3. Selected Bond Distances (Å) and Angles (deg) for
8b, 8c, and 9c
2
2
2
8
b
8c
9c
M−O(1)
M−O(2)
M−O(3)
M−O(4)
2.005(2)
2.019(3)
2.0117(18)
2.0091(18)
2.0128(19)
2.0072(19)
2.399(2)
2.007(3)
2.008(3)
To better understand the decisive factors for the formation of
2.025(3)
2.004(2)
4
a and 5a, parallel studies were performed employing H [1b]
2
2.028(3)
1.995(2)
and H [1c] (Scheme 4). In particular, no ligand degradation
2
M−N(1)
2.460(3)
2.356(3)
was found in these reactions. The preparation of 8b requires
prolonged heating at 80 °C, whereas that of 8c proceeds
smoothly even at ambient conditions. No reaction occurs when
a toluene-d solution of 2b with equimolar H [1b] was stirred
M−N(2)
2.449(3)
2.393(2)
2.365(3)
O(1)−M−O(2)
O(1)−M−O(3)
O(1)−M−O(4)
O(1)−M−N(1)
O(1)−M−N(2)
O(2)−M−O(3)
O(2)−M−O(4)
O(2)−M−N(1)
O(2)−M−N(2)
O(3)−M−O(4)
O(3)−M−N(1)
O(3)−M−N(2)
O(4)−M−N(1)
O(4)−M−N(2)
N(1)−M−N(2)
158.78(10)
90.89(10)
91.16(11)
79.82(10)
104.90(11)
93.87(10)
91.88(11)
79.12(10)
96.30(10)
158.60(10)
96.26(10)
78.78(10)
105.07(10)
80.11(10)
173.09(10)
158.58(8)
95.97(8)
159.90(11)
95.84(11)
90.09(11)
79.44(11)
97.00(10)
90.37(11)
90.54(11)
80.89(11)
102.92(11)
159.95(11)
96.18(11)
79.61(10)
103.74(11)
80.66(10)
174.26(10)
8
2
90.66(8)
1
at room temperature for 24 h ( H NMR evidence). The
formation rate of 8b is thus significantly less than that of 8c,
reflecting unambiguously the steric rather than electronic
difference of these isomeric N-substituents. Interestingly,
though 9c could be synthesized quantitatively with conditions
similar to those of 8c (25 °C, 16 h), attempts to prepare 9b
under even rather harsher conditions (80 °C, 4 days) led to the
78.99(7)
97.00(8)
90.19(8)
91.05(8)
79.85(7)
104.33(8)
158.42(8)
97.80(7)
formation of a mixture containing 3b and the presumed 9b
1
(
judged by H NMR) in a ratio of >10:1. This result is
78.92(8)
surprising but can be ascribed to an activation barrier for 3b
conversion to 9b significantly higher than that for 2b
conversion to 8b. Though rarely encountered, hafnium
complexes have in some cases exhibited thermal stability and
103.62(8)
79.91(8)
174.60(7)
5
3
chemical inertness greater than those of their 4d congeners
because of inherently stronger metal−ligand bond strengths. In
the present study, we propose that the Hf−OiPr bonds in 3b
homoleptic complexes, containing two meridional [ONO]2−
ligands with the two nitrogen atoms being trans-disposed.
Similar structural features were also reported by Kol, Gold-
schmidt, and co-workers for homo- and heteroleptic ONO
are relatively inert; thus, their protonation with H [1b]
2
becomes more challenging and less effective. These homoleptic
54
complexes of titanium. Davidson and co-workers also
complexes are all thermally stable at elevated temperatures (19
reported independently a series of structurally characterized
homoleptic group 4 complexes of sterically less demanding
1
mM in toluene-d , 80 °C, 24 h, H NMR evidence). In contrast
8
55
to the mono-ONO-ligated 4a, the bis-ligated 8c is inactive for
ROP catalysis under similar conditions or in the presence of
additional benzyl alcohol.
ONO ligands. Though complexes presented herein are
essentially isostructural to one another, the M−N bond
distances in 8b are slightly longer than those in 8c and 9c.
At first glance, this phenomenon appears irrational as both n-
propyl and isopropyl carry similar electronic characteristics and
the steric repulsion between two N-bound alkyls should be
negligible in view of trans-disposed nitrogen donors. Structural
scrutiny of these molecules revealed that these N-bound alkyls
have marginally close contacts with one of the ortho-tert-butyl
groups on the phenolate rings. A representative drawing is
provided in Figure S5 (see Supporting Information), high-
lighting the steric interaction between the N2-bound isopropyl
and the spatially peripheral tert-butyl group ortho to O1 in 8b.
With less sterically hindered n-propyl groups in 8c and 9c, the
The solution NMR data of 8b, 8c, and 9c are all indicative of
a structure having time-averaged C symmetry. For instance,
2
1
four distinct singlet resonances are observed in the H NMR
spectra for aryl tert-butyl groups in these molecules. The
benzylic methylene moieties are all diastereotopic, so are the N-
isopropyl methyl groups in 8b and the N-CH CH Me in 8c and
2
2
9
c. Colorless crystals of 8b, 8c, and 9c suitable for X-ray
diffraction analysis were grown from concentrated pentane
solutions at −35 °C. Figure 2 depicts the molecular structures
of these complexes. Table 3 summarizes selected bond
distances and angles. As illustrated, these compounds are
E
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Inorganic Chemistry
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nitrogen donors thus lie closer to the metal center than those in
isopropyl-derived 8b.
Parallel studies employing either isopropyl- or n-propyl-derived
ONO ligands gave different results, wherein no ligand
degradation was observed. These results are intriguing as the
latter ligands are close, lower homologues of the former. The
formation rates of homoleptic 8b and 9b in comparison with
those of 8c and 9c highlight the steric effect of these ONO
ligands, in spite of the fact that the N-alkyl substituents in these
ligands are isomeric. Though produced unexpectedly by a
The divergent preferences for the formation of 4a and 5a
versus 8b−c and 9b−c deserve more comments. Though 8b−c
and 9b−c are all bis-ligand complexes, the preparation of
isopropyl-derived 8b and 9b requires conditions to acquire
reasonable synthetic yields much more severe than those of n-
propyl-substituted 8c and 9c for steric reasons. As mentioned
previously, the steric argument is also supported by X-ray
structural data of 8b, 8c, and 9c, from which close contacts are
found, though marginally, for N-alkyls with spatially peripheral
ortho-tert-butyl substituents on the phenolate rings. With more
sterically demanding tert-butyl groups at the nitrogen donors,
process involving thermal degradation of H [1a], 4a is an active
2
catalyst for ε-caprolactone polymerization, underscoring the
fact that the presumed innocent ligands may under certain
circumstances become reactive, providing perhaps misleading
conclusions in catalysis. In sharp contrast, the bis-ligated
complex 8c is inactive for ROP catalysis. The comparison in
ROP activities of 4a and 8c is intriguing, particularly from the
standpoint that both complexes are prepared with identical
synthetic strategies. These findings are informative and relevant
to the structural and reaction chemistry of complexes
containing the conceivably ubiquitous Mannich-type ligands.
the presumed bis-ligand M[1a] (M = Zr (8a) or Hf (9a))
2
should be sterically unfavorable and synthetically rather
inaccessible. In view of the synthetic conditions employed for
8
b−c and 9b−c, the preparation of the presumed 8a and 9a
would require even harsher conditions, that is, prolonged
heating at temperatures much higher than 80 °C. On the basis
of the reaction conditions employed for the preparation of 4a
and 5a, the presumed energies of 8a and 9a are apparently
higher than those of 4a and 5a, thus precluding the possibility
of 4a and 5a evolution from 8a and 9a.
EXPERIMENTAL SECTION
■
General Procedures. Unless otherwise specified, all experiments
were performed under nitrogen using standard Schlenk or glovebox
techniques. All solvents were reagent grade or better and purified by
standard methods. The NMR spectra were recorded on Varian Unity
or Bruker AV instruments. Chemical shifts (δ) are listed as parts per
million downfield from tetramethylsilane. Coupling constants (J) are
The bis-ligand complexes 8b−c and 9b−c were prepared
from alcoholysis reactions of 2b−c and 3b−c, respectively, with
their corresponding ligand precursors H [1b−c]. It is
2
reasonable to propose that these reactions proceed via
1
reversible H [1b−c] coordination, either O-bound or N-
listed in hertz. H NMR spectra are referenced using the residual
2
13
bound, to the metal center of 2b−c or 3b−c followed by
proton transfer between oxygen donors to liberate isopropanol
and generate 8b−c and 9b−c. In view of the Lewis basic
characteristics of O and N, the N-bound intermediacy appears
more favorable. This hypothesis is also consistent with the
observed formation rates of 8b and 9b in comparison with
those of 8c and 9c. With more sterically demanding tert-butyl
solvent peak at δ 7.16 for C
6 6
D . C NMR spectra are referenced using
the internal solvent peak at δ 128.39 for C D . The assignment of the
6
6
carbon atoms for all new compounds is based on distortionless
enhancement by polarization transfer 13C NMR spectroscopy. All
NMR spectra were recorded at room temperature in specified solvents
unless otherwise noted. Elemental analysis was performed on a
Heraeus CHN-O rapid analyzer. Gel permeation chromatography
analyses were carried out at 45 °C on a JASCO instrument equipped
with two Waters Styragel HR columns in series and a JASCO RI-2031
refractive index detector. HPLC grade THF was supplied at a constant
flow rate of 1.0 mL/min with a JASCO PU-2080 isocratic HPLC
substituents in H [1a], 2a, and 3a, a similar N-bound
2
intermediacy may be even more unfavorable on steric grounds,
though it may conceptually assist H [1a] degradation.
2
Interestingly, ligand degradation does not occur for H [1b]
pump. Molecular weights (M and M ) were determined by
n w
2
interpolation from calibration plots established with polystyrene
standards.
and H [1c], even in the presence of Lewis acidic metal
2
complexes at elevated temperatures, for which the relatively
weaker electron-releasing nature of the N-donor in these ligand
precursors should be responsible.
4
0
40
54
Materials. Compounds H [1a], H [1b], and H [1c] were
2
2
2
prepared according to literature procedures. All other chemicals were
obtained from commercial vendors and used as received.
As discussed above, we prefer the justification illustrated in
X-ray Crystallography. Crystallographic data for all structurally
characterized complexes are available in Supporting Information. Data
were collected on a Bruker-Nonius Kappa CCD diffractometer with
graphite monochromated Mo Kα radiation (λ = 0.7107 Å). Structures
were solved by direct methods and refined by full matrix least-squares
procedures against F2 using SHELXL-97. All full-weight non-
hydrogen atoms were refined anisotropically. Hydrogen atoms were
placed in calculated positions. The structures of 5a, 8b, 8c, and 9c
contain disordered pentane. Attempts to obtain suitable disorder
Scheme 3 for the formation of 4a and 5a. Note that H [1a]
2
degrades in solutions at ambient temperature in the presence or
absence of 2a or 3a. Upon thermal decomposition of H [1a],
2
the produced o-QM 7 reacts either with other unsaturated
molecules, including 7 itself, present in solutions to give
cycloaddition products, or with 2a or 3a to produce the
structurally characterized ethereal phenolate complex 4a or 5a,
respectively.
56
57
models failed. The SQUEEZE procedure of the Platon program was
2
used to obtain a new set of F (hkl) values for each structure without
CONCLUSIONS
■
the contribution of solvent molecules, leading to the presence of
significant voids in these structures. The refinement reduced R1 values
of 5a, 8b, 8c, and 9c to 0.0235, 0.0667, 0.0526, and 0.0342,
respectively. Structures reported herein also contain disordered tert-
butyl groups in ONO or methyl groups in isopropoxide that could not
be resolved properly, thus giving rise to B-level alerts in checkCIF
reports. CCDC entries 910754 (for 4a), 910755 (for 5a), 910756 (for
We have demonstrated that the thermal stability of amine
biphenol compounds is a function of their incorporated N-
substituents. With a tert-butyl group at the nitrogen donor, the
ONO ligand H [1a] is susceptible to thermal degradation,
2
unexpectedly producing a transient o-QM derivative that, under
the conditions employed, reacts subsequently with metal
alkoxides to afford a novel ethereal phenolate ligand as
observed in complexes 4a and 5a. As a result, these reactions
involve both C−N bond cleavage and C−O bond formation.
8
b), 910757 (for 8c), and 910758 (for 9c) contain the supplementary
crystallographic data for this Article. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam. ac.uk/data_request/cif.
F
dx.doi.org/10.1021/ic400891b | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Synthesis of Zr[1a](OiPr)(iPrOCH -2-O-3,5-C H (tBu) ) (4a).
NCH H CH Me), 1.56 (s, 18, ArCMe ), 1.44 (s, 18, ArCMe ), 1.37
2
6
2
2
E
F
2
3
3
To a toluene solution (6 mL) of H [1a] (200 mg, 0.392 mmol) was
(s, 18, ArCMe ), 1.25 (br m, 4, NCH CH Me), 1.22 (s, 18, ArCMe ),
2
3 2 2 3
13
added a toluene solution (6 mL) of Zr(OiPr) (HOiPr) (76 mg, 0.196
0.55 (t, 6, J = 7.3, NCH CH Me). C NMR (C D , 125 MHz) δ:
2 2 6 6
4
mmol) at room temperature. The solution was heated with stirring in
an oil bath at 80 °C for 16 h, filtered through a pad of Celite, and
evaporated to dryness under reduced pressure. The solid residue was
washed with acetonitrile (6 mL) to afford the product as an off-white
159.4 (C), 158.9 (C), 140.7 (C), 140.1 (C), 136.6 (C), 135.6 (C),
125.6 (CH), 125.1 (CH), 124.6 (CH), 124.5 (CH), 124.3 (C), 122.6
(C), 59.7 (ArCH N), 59.1 (ArCH N), 49.6 (NCH CH Me), 35.4
2
2
2
2
(ArCMe ), 35.3 (ArCMe ), 34.8 (ArCMe ), 34.6 (ArCMe ), 32.4
3
3
3
3
1
solid. Yield: 98 mg (52%). H NMR (C D , 500 MHz) δ: 7.50 (d, 2, J
(ArCMe ), 32.3 (ArCMe ), 30.9 (ArCMe ), 30.7 (ArCMe ), 13.2
6
6
3
3
3
3
=
2.1, ArH), 7.49 (d, 1, J = 2.3, ArH), 7.15 (d, 2, J = 2.1, ArH), 6.68 (d,
(NCH
2
CH
2
Me), 11.7 (NCH CH Me). Anal. Calcd for
2 2
1
, J = 2.1, ArH), 4.79 (sept, 1, J = 6.1, ZrOCHMe ), 4.69 (br s, 2,
C
66
H
102
N
O
2
4
Zr: C, 73.47; H, 9.54; N, 2.60. Found: C, 73.41; H,
9.57; N, 2.62.
Synthesis of Hf[1c]
followed for 8c except employing Hf(OiPr)
Zr(OiPr) (HOiPr), affording the product as an off-white solid. Yield:
99%. H NMR (C , 500 MHz) δ: 7.57 (d, 2, J = 2.4, ArH), 7.42 (d,
2, J = 2.4, ArH), 7.14 (d, 2, J = 2.4, ArH), 7.11 (d, 2, J = 2.4, ArH), 5.18
2
ArCH OCHMe ), 4.43 (sept, 1, J = 6.5, ArCH OCHMe ), 4.14 (d, 2,
2
2
2
2
J = 14.4, ArCH H N), 3.58 (d, 2, J = 14.5, ArCH H N), 1.90 (s, 9,
ArCMe ), 1.60 (s, 18, ArCMe ), 1.38 (s, 18, ArCMe ), 1.37 (br s, 6,
ZrOCHMe ), 1.30 (s, 9, ArCMe ), 1.17 (s, 9, NCMe ), 0.74 (d, 6, J =
2
(9c). The procedures were similar to those
(HOiPr) in the place of
A
B
A
B
3
3
3
4
2
3
3
4
13
1
6
1
1
.5, ArCH OCHMe ). C NMR (C D , 125 MHz) δ: 159.2 (C),
58.5 (C), 140.8 (C), 140.3 (C), 137.5 (C), 137.3 (C), 129.7 (C),
29.6 (C), 124.1 (CH), 123.9 (CH), 123.5 (CH), 122.4 (CH), 75.9
D
6 6
2
2
6
6
(d, 2, J = 13.4, ArCH
J = 12.9, ArCH ), 3.50 (d, 2, J = 13.4, ArCH
NCH CH Me), 2.79 (m, 2, NCH CH
ArCMe ), 1.46 (s, 18, ArCMe ), 1.37 (s, 18, ArCMe
4, NCH CH Me), 1.22 (s, 18, ArCMe ), 0.54 (t, 6, J = 7.3,
NCH CH Me). C NMR (C , 125 MHz) δ: 159.4 (C), 159.0 (C),
140.5 (C), 139.8 (C), 137.1 (C), 136.0 (C), 125.5 (CH), 124.9 (CH),
124.7 (CH), 124.6 (CH), 124.1 (C), 122.4 (C), 59.6 (ArCH N), 58.8
(ArCH N), 49.9 (NCH CH Me), 35.4 (ArCMe ), 35.2 (ArCMe ),
34.8 (ArCMe ), 34.6 (ArCMe ), 32.4 (ArCMe ), 32.3 (ArCMe ), 30.8
(ArCMe ), 30.7 (ArCMe ), 13.2 (NCH CH Me), 11.8
(NCH CH Me). Anal. Calcd for C66 : C, 67.97; H, 8.82;
H
B
), 4.91 (d, 2, J = 12.9, ArCH
H
), 3.56 (d, 2,
), 3.02 (m, 2,
B
A
C
D
(
(
(
(
(
ArCH OCHMe ), 74.0 (ZrOCHMe ), 67.4 (ArCH OCHMe ), 62.7
NCMe ), 61.9 (ArCH N), 36.0 (ArCMe ), 35.6 (ArCMe ), 34.7
ArCMe ), 34.6 (ArCMe ), 32.4 (ArCMe ), 32.3 (ArCMe ), 30.9
ArCMe ), 30.8 (ArCMe ), 27.7 (NCMe ), 21.8 (OCHMe ), 1.8
C
H
D
A
H
2
2
2
2
2
H
H
Me), 1.57 (s, 18,
3
2
3
3
E
F
2
E
F
2
), 1.25 (br m,
3
3
3
3
3
3
3
3
3
3
2
2
2
3
13
ArCH OCHMe ). Anal. Calcd for C H NO Zr: C, 70.60; H, 9.59;
2
2
D
6 6
2
2
55 89
5
N, 1.50. Found: C, 69.98; H, 9.53; N, 1.53.
Synthesis of Hf[1a](OiPr)(iPrOCH -2-O-3,5-C H (tBu) ) (5a).
The procedures were similar to those followed for 4a except
employing Hf(OiPr) (HOiPr) in the place of Zr(OiPr) (HOiPr),
2
6
2
2
2
2
2
2
3
3
4
4
3
3
3
3
1
affording the product as an off-white solid. Yield: 48%. H NMR
3
3
2
2
(
C D , 500 MHz) δ: 7.51 (br s, 2, ArH), 7.50 (br s, 1, ArH), 7.13 (br s,
2
2
H102HfN O
2 4
6
6
2
, ArH), 6.65 (br s, 1, ArH), 4.87 (sept, 1, J = 6.0, HfOCHMe ), 4.66
N, 2.40. Found: C, 68.15; H, 8.55; N, 2.25.
2
(
4
br s, 2, ArCH OCHMe ), 4.47 (sept, 1, J = 6.2, ArCH OCHMe ),
2
2
2
2
.14 (d, 2, J = 14.2, ArCH H N), 3.57 (d, 2, J = 14.5, ArCH H N),
ASSOCIATED CONTENT
Supporting Information
A
B
A
B
■
1
.89 (s, 9, ArCMe ), 1.59 (s, 18, ArCMe ), 1.38 (s, 18, ArCMe ), 1.36
3
3
3
*
S
(
(
(
br s, 6, HfOCHMe ), 1.31 (s, 9, ArCMe ), 1.18 (s, 9, NCMe ), 0.73
d, 6, J = 6.0, ArCH OCHMe ). C NMR (C D , 125 MHz) δ: 158.9
C), 158.6 (C), 140.7 (C), 140.2 (C), 138.1 (C), 138.1 (C), 129.7 (C),
29.3 (C), 124.2 (CH), 123.8 (CH), 123.5 (CH), 122.1 (CH), 76.1
ArCH OCHMe ), 73.3 (HfOCHMe ), 67.5 (ArCH OCHMe ), 63.2
2 3 3
13
X-ray crystallographic data for all structurally characterized
2
2
6
6
1
compounds, H NMR spectrum of PCL prepared, and mass
1
spectra and kinetic data of thermolysis of H [1a]. This material
is available free of charge via the Internet at http://pubs.acs.org.
2
(
(
(
2
2
2
2
2
NCMe ), 61.8 (ArCH N), 35.9 (ArCMe ), 35.6 (ArCMe ), 34.7
ArCMe ), 34.6 (ArCMe ), 32.5 (ArCMe ), 32.4 (ArCMe ), 30.9 (br,
3
2
3
3
3
3
3
3
AUTHOR INFORMATION
Corresponding Author
E-mail: lcliang@mail.nsysu.edu.tw.
■
*
ArCMe ), 27.9 (NCMe ), 21.7 (OCHMe ), 1.8 (ArCH OCHMe ).
Anal. Calcd for C H HfNO : C, 64.57; H, 8.77; N, 1.37. Found: C,
3
3
2
2
2
55
89
5
6
4.60; H, 8.87; N, 1.36.
Synthesis of Zr[1b] (8b). The procedures were similar to those
2
Notes
followed for 4a except employing H [1b] in the place of H [1a] and a
reaction time of 24 h, affording the product as an off-white solid. Yield:
2
2
The authors declare no competing financial interest.
1
4
2
2%. H NMR (C D , 500 MHz) δ: 7.51 (d, 2, J = 2.4, ArH), 7.47 (d,
6 6
ACKNOWLEDGMENTS
, J = 2.4, ArH), 7.18 (d, 2, J = 2.3, ArH), 7.09 (d, 2, J = 2.3, ArH), 5.28
■
(
d, 2, J = 10.8, ArCH H ), 5.25 (d, 2, J = 10.7, ArCH H ), 3.66 (d, 2, J
We thank the National Science Council of Taiwan for financial
support (NSC 99-2113-M-110-003-MY3 and 99-2119-M-110-
002), Mr. Ting-Shen Kuo (NTNU) for assistance with X-ray
crystallography, and the National Center for High-Performance
Computing (NCHC) for the access to chemical databases.
A
B
A
B
=
13.7, ArCH H ), 3.49 (s, 2, J = 13.8, ArCH H ), 3.39 (sept, 2, J =
C D C D
6
1
1
1
1
.8, NCHMe ), 1.48 (s, 18, ArCMe ), 1.42 (s, 18, ArCMe ), 1.37 (s,
2 3 3
8, ArCMe ), 1.36 (s, 18, ArCMe ), 1.22 (d, 6, J = 6.8, NCHMe Me ),
3
3
A
B
13
.06 (d, 6, J = 6.8, NCHMe Me ). C NMR (C D , 125 MHz) δ:
A
B
6
6
59.6 (C), 159.4 (C), 140.9 (C), 140.6 (C), 136.5 (C), 136.0 (C),
26.1 (C), 125.7 (C), 125.4 (br, CH), 124.6 (br, CH), 60.3
REFERENCES
(
(
(
(
ArCH N), 59.9 (ArCH N), 51.4 (NCHMe ), 35.9 (ArCMe ), 35.2
ArCMe ), 34.7 (ArCMe ), 34.6 (ArCMe ), 32.8 (ArCMe ), 32.3
ArCMe ), 31.7 (ArCMe ), 30.8 (ArCMe ), 18.9 (NCHMe Me ), 18.1
NCHMe Me ). Anal. Calcd for C H N O Zr: C, 73.47; H, 9.54;
■
2
2
2
3
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3
3
3
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4
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.53 (d, 2, J = 2.2, ArH), 7.39 (d, 2, J = 2.2, ArH), 7.15 (d, 2, J = 2.2,
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E
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dx.doi.org/10.1021/ic400891b | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
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