Synthesis, characterization, and oxidation of new POCNimine-type pincer complexes of nickel

Article abstract of DOI:10.1021/om500529e

This report describes the synthesis and characterization of new pincer-type complexes of divalent and trivalent nickel. Refluxing toluene mixtures of NiBr₂(CH₃CN)_n and the unsymmetrical pincer-type ligands Ph-POC^H

Full text of DOI:10.1021/om500529e

Article
pubs.acs.org/Organometallics
Synthesis, Characterization, and Oxidation of New POCNimine-Type
Pincer Complexes of Nickel
Berline Mougang-Soume, Francine Belanger-Gariepy, and Davit Zargarian*
́
́
Department of Chemistry, Universite
́
de Montrea
́
l, Montreal, Quebec, Canada
S
* Supporting Information
ABSTRACT: This report describes the synthesis and
characterization of new pincer-type complexes of divalent
and trivalent nickel. Refluxing toluene mixtures of
NiBr2(CH3CN)n and the unsymmetrical pincer-type ligands
Ph-POCHNimine-R gave (Ph-POCNimine-R)Ni(II)Br in good
yields (Ph-POCHNimine-R = 1-(Ph2PO),3-(CHNR)C6H4; R =
Bn, 1; Ph, 2; t-Bu, 3; Cy, 4). The C−H nickelation step
involved in these syntheses is facilitated in the presence of
NEt3. Treating compounds 1−4 with Br2, I2, N-bromosucci-
nimide (NBS) and N-chlorosuccinimide appears to bring
about the desired one-electron oxidation of the Ni(II) center
in these complexes, but the putative trivalent species
decompose over time and could not be isolated. One of the decomposition products obtained in the Br2 reaction was
identified as the zwitterionic compound [Br3Ni(II){κO-(E)-N-(3-((diphenylphosphoryl)oxy)benzylidene)benzenaminium}], 5.
To identify the factors that are important for isolation of stable trivalent derivatives, we prepared the divalent precursors (i-Pr-
POCNimine-Ph)NiX (X= Br, 6, and NCS, 9) and studied their reactions with various oxidants. Whereas treatment of 6 with Br2 or
NBS gave the target 17-electron complex (i-Pr-POCNimine-Ph)Ni(III)Br2 (7), no mixed-ligand trivalent complexes of the type (i-
Pr-POCNimine-Ph)Ni(III)Br(X) could be isolated from reactions of (i-Pr-POCNimine-Ph)Ni(NCS) (9) with NBS or of 6 with N-
chlorosuccinimide. The latter reaction gave a complex mixture from which was isolated a paramagnetic, trinuclear compound (8)
composed of octahedral Ni(II) units featuring μ2-Cl and μ2,κO,κN-succinimide fragments. The solid state structures of all new
complexes 2-9 have been elucidated by X-ray crystallography.
based on a 2,6-disubstituted phenylene backbone to encompass
a large variety of terdentate, monoanionic ligands featuring N-,
P-, O-, S-, or C-based donor moieties and different backbones
(aliphatic,3 aromatic,4 symmetrical,3a,4a−h,5 unsymmetrical6).
Rigid, planar backbones generally give thermally robust metal
complexes that often display interesting catalytic,7 materials,8
and photophysical properties.7b,9 A combination of such
functional properties and the relative ease with which the
steric and electronic properties of pincer complexes can be
modified have contributed to the growing popularity of ECE-
type pincer complexes.10
INTRODUCTION
■
Most of the early ECE-type pincer complexes reported in the
1970s by the groups of Shaw1 and van Koten2 had the general
formula {2,6-(ECH2)2C6H3}MLn wherein E represented a
neutral donor moiety such as PR2 or NR2 (Chart 1). Over the
last four decades, the molecular architecture of ECE-type pincer
ligands has evolved from the prototypical symmetrical structure
Chart 1. Structures and Acronyms for Various Pincer
Systems
In this context, the reactivities of organonickel complexes
featuring symmetrical PCP and POCOP ligands based on
aromatic as well as aliphatic backbones (i and ii in Chart 1)
have received much attention over the past two decades.3i−m,11
Our group has also introduced nickel complexes of unsym-
metrical pincer ligands featuring 2,6-disubstituted phenylene12
and 1,3-indenylene13 backbones (iii, iv, and v in Chart 1). We
have also described the synthesis via C−H nickelation of a first
series of POCNamine-type complexes (vi in Chart 1) and briefly
explored their reactivities in hydroalkoxylation and Kharasch
Received: May 16, 2014
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of Ligands e−h and Complexes 1−4
additions.14 Studies conducted on the latter complexes showed
that the N-substituents influence the redox behavior of the Ni
center, and attempted oxidation of the benzylamine derivative
{2-OP(i-Pr2),6-CH2NH(Bn)-C6H3}NiBr (Bn = CH2Ph) gave
instead the first example of a POCNimine-type complex14c (vii in
Chart 1).
Whereas the influence of some structural variations on
reactivities of PCP−Ni and POCOP−Ni complexes has been
studied extensively,3j,l,11 very few studies have explored the
impact of N- and P-substituents on the structures and
reactivities of POCN−Ni complexes. Thus, we have reported
on the syntheses and reactivities of (i-Pr-POCNamine)NiBr
complexes featuring four different amine moieties (NMe2,
NEt2, morpholino, and N(H)Bn).14a,b We have also reported
the preparation and oxidation of one POCNimine−Ni complex,
(i-Pr-POCNimine-Bn)NiBr,14c whereas Xia’s group has reported
leading to the new complexes 1−4 is shown in Scheme 1. The
iminophenol preligands a−d were prepared by condensation of
3-hydroxybenzaldehyde with the corresponding primary
amines.14c,23 The preligands were isolated in good yields as
powders that were white (a), brown (b), yellow (c), or beige
(d), and their identities were established by elemental analysis
and NMR spectroscopy. The most characteristic signals were
those of the imine and phenol protons (ca. 8.2−8.3 and 5.0−
5.1 ppm, respectively) and the imine carbon (ca. 162−163
ppm).
Phosphination of a−d with ClPPh2 and NEt3 gave the
corresponding Ph-POCNimine ligands e−h, which were isolated
in high yields as colorless (e), brown (f), or yellowish (g, h) oils
and characterized by NMR spectroscopy (e.g., 31P δ: 110−
112). It is important to note that all of these ligands are air
sensitive and must be stored under inert atmosphere. A
PARATONE oil coated single crystal of f proved sufficiently
stable to be subjected to X-ray diffraction analysis; the ORTEP
diagram for this compound is shown in Figure 1 and will be
discussed below along with the solid state structures of 2−4.
Heating the ligands e−h with NiBr2(MeCN)n in the
presence of NEt3 (toluene, 110 °C, 2−4 h) led to ligand
nickelation and furnished the new complexes as brick-red (2)
or orange (1, 3, and 4) solids in 72−95% yields (Scheme 1). It
is worth emphasizing that the nickelation step does proceed in
the absence of added base, but using NEt3 maximizes the
reaction yield in all cases since it quenches the HBr generated
during the cyclometalation step, thus preventing it from
protonating the donor moieties of the ligands.24
the syntheses and Kumada coupling activities of (R-POCNimine
-
Ar)Ni(II) complexes (R = Ph, and i-Pr; Ar = 2,6-i-Pr2-C6H3).15
This paucity of studies/data prompted us to prepare a new
series of (R-POCNimine)Ni(II)Br complexes bearing different P-
and N-substituents and explore their structures, reactivities, and
oxidation potentials.
Presented herein are the synthesis and characterization of the
new complexes (Ph-POCNimine-R)NiBr (R = Bn (1); Ph (2); t-
Bu (3); Cy (4)) and (i-Pr-POCNimine-Ph)NiX (X = Br (6),
NCS (9)). This report also describes the successful isolation of
the trivalent complex (i-Pr-POCNimine-Ph)NiBr2 (7) as well as
various unsuccessful oxidation attempts that led to isolation of
two unusual Ni(II) compounds, the zwitterionic tetrahedral
species [Br3Ni(II){κO-(E)-N-(3-((diphenylphosphoryl)oxy)-
benzylidene)benzenaminium}], 5, and the trinuclear species
composed of octahedral Ni(II) units featuring μ2-Cl and
μ2,κO,κN-succinimide fragments. Our interest in trivalent nickel
complexes stems from the important role that species bearing
Ni(III)−C bonds are believed to play in hydrocarbon
oxidation16 and coupling reactions.17,18 Mechanistic proposals
for the latter transformations often invoke five-coordinate alkyl-
or aryl-bound di- or trivalent intermediates. While many
pentacoordinated organometallic Ni(II) species are known,19
far fewer examples of their trivalent counterparts have been
reported. Indeed, until very recently, most examples of fully
characterized organometallic Ni(III) species were based on
NCN-type pincer ligands popularized by van Koten’s group,20
but a number of recent reports have introduced trivalent
complexes based on different ligand frames.17,21,22 Therefore,
the isolation and characterization of NiIII species bearing one or
more Ni−C bonds is critical in efforts to expand organonickel
chemistry.
All derivatives (Ph-POCNimine)NiX are air stable: indefinitely
in the solid state and over short time periods in solution.
Indeed, some derivatives were purified by extracting their
solutions briefly with water to remove residual traces of salts.
The bromo derivatives 1−4 also show good thermal stabilities
in the solid state, melting or decomposing above 220 °C,
whereas their solutions in hexane, toluene, benzene, acetoni-
trile, or dichlorometane are stable at rt over several hours.
The diamagnetic complexes 1−4 were fully characterized by
NMR spectroscopy (1H, 13C{1H}, 31P{1H}, COSY, HSQC,
and HMBC). The 31P{1H} NMR spectra showed singlet
resonances at 141−150 ppm, 30−40 ppm downfield of the
resonances for the corresponding ligands (ca. 110−112 ppm).
The 1H and 13C{1H} NMR spectra of 1−4 were quite complex
due to the presence in these molecules of multiple aromatic
1
moieties, but most signals were assigned thanks to H−COSY,
HSQC, and HMBC correlation experiments. The CHNR
proton was most distinctive both for its characteristic chemical
shift (ca. 8.0−8.5 ppm) and its multiplicity due to a 4-bond
coupling to the 31P nucleus. For example, a doublet is observed
for this proton (4JP−H = 4 Hz) in complexes 2 (CHNPh) and 3
(CHNBut), whereas broadened or more complex multiplicities
were observed in NCH2Ph derivatives due to additional
RESULTS AND DISCUSSION
■
Synthesis and Spectroscopic Characterization of (Ph-
POCNimine-R)Ni(II)Br Complexes 1−4. The synthetic route
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Figure 1. ORTEP diagrams for ligand f (a) and complexes 2 (b), 3 (c), and 4 (d). Thermal ellipsoids are set at the 50% probability level. Most
hydrogen atoms are omitted for clarity. A selection of structural parameters is listed in Table 1. Selected bond distances (Å) and angles (deg) for
ligand f: P−O = 1.671(1); P1−C14 = 1.826(2); P1−C20 = 1.846(2); N−C7 = 1.276(3); N−C8 = 1.424(3); P1−O1−C2 = 120.2(1); O1−P1−C14
= 96.35(8); O1−P1−C20 = 100.93(8); P1−O1−C2−C1 = 150.1(1); C1−C6−C7−N1 = 169.0(2).
coupling to the benzylic protons. Similar observations were
made in the 13C{1H} NMR spectra for the imine carbon
nucleus.
Table 1. Selected Bond Distances (Å) and Angles (deg) for
Complexes 2−4
parameter
2
3
4
Solid State Structures of Ligand f and Complexes 2−
4. Figure 1 shows the ORTEP diagrams for ligand f and
complexes 2−4, and selected structural parameters are listed in
the figure caption (for f) and Table 1 (2−4); Table S1 in the
Supporting Information provides the crystal data. Comparison
of structural parameters in f and 2 reveals the impact of
complexation to Ni. For instance: the phosphinite and imine
moieties adopt a transoid disposition in f but are cisoid in 2;
orientation of the N-Ph ring with respect to the central aryl
moiety is 14° in f and 49° in 2; there is a tightening in the
angles P−O−Ar (ca. 120° vs 109°) and N−C−Ar (ca. 122° vs
116°); the P−O distance shrinks (1.671(1) vs 1.645(1) Å)
while the N−C7 lengthens (1.276(3) vs 1.293(2) Å).
Ni−C(1)
Ni−P(1)
Ni−N(1)
Ni−X(1)
1.856(1)
2.1099(5)
1.994(1)
2.3358(3)
1.645(1)
1.293(2)
1.433(2)
80.20(5)
82.55(6)
171.57(5)
162.25(4)
101.61(4)
96.03(1)
1.859(2)
2.1231(5)
2.050(1)
2.3705(3)
1.657(1)
1.292(2)
1.497(2)
79.97(5)
82.47(6)
170.30(5)
162.27(4)
106.87(4)
90.78(2)
1.863(2)
2.1192(6)
1.988(2)
2.3518(4)
1.658(2)
1.278(3)
1.477(3)
80.04(6)
82.23(8)
173.37(6)
160.73(5)
101.70(5)
96.69(2)
P(1)−O(1)
C(7)N(1)
C(8)−N(1)
C(1)−Ni−P(1)
C(1)−Ni−N(1)
C(1)−Ni−X(1)
N(1)−Ni−P(1)
N(1)−Ni−X(1)
P(1)−Ni−X(1)
The overall geometry around the nickel center in 2−4 is a
distorted square planar arrangement as defined by the atoms
C1, P1, N1, and Br. The carbon atoms C7 and C8 bonded to
the imine N atom are also in the main plane due to the π-
delocalization in the POCN ligand. The main source of
structural distortions is the small bite angle of the pincer ligand
(N1−Ni−P1 ∼ 161−163°) that is also responsible for the
smaller- and larger-than-90° cis angles (e.g., C1−Ni−P1 ∼ 80°;
N1−Ni−X ∼ 102−107°). The Ni−C1 bond lengths of 1.86−
1.87 Å are comparable to the corresponding distances observed
C
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in the complex i-Pr-POCNimine (1.86 Å for R = Bn),14c but
benzenaminium}], 5, a zwitterionic tetrahedral complex
composed of a nickelate anion and an iminium cation (Scheme
2, Figure 2). Thus, instead of the intended single-electron
shorter than those of POCNamine-type complexes (2.03 Å).14a
The impact of different N-substituents R in (Ph-POCNimine
-
R)NiBr is evident when we compare the structures of 3 (N-t-
Bu), 2 (N-Ph), and 4 (N-Cy): complex 3 displays the longest
Ni−Br distance (2.37 vs 2.34−2.35 Å) and the largest N−Ni−
Br angle (107° vs 102°), presumably due to the greater steric
demands of the t-Bu-N moiety. The same factor might also
explain the much longer Ni−N distance in 3 vs 2 and 4: ca.
2.05 vs 1.99 Å. Curiously, the Ni−P bond distances also
increase with the increasing steric bulk of the N-substituent,
2.1099(5) Å for N-Ph < 2.1192(6) Å for N-Cy < 2.1231(5) Å
for N-t-Bu, which seems counterintuitive, because we would
expect that the longer (and presumably weaker) Ni−N bond
should lead to stronger and shorter Ni−P bonds. This
anomalous observation can be rationalized by considering
that the greater steric bulk at the imine moiety displaces the Br
atom toward the phosphinite moiety, which would in turn
lengthen/weaken the Ni−P bond. Indeed, the N−Ni−Br and
P−Ni−Br angles bear witness to this phenomenon: N−Ni−Br
is largest and P−Ni−Br is smallest for the t-BuN derivative 3
(Table 1).
Scheme 2. Attempted Oxidation of Complex 2
Synthesis and Characterization of Trivalent Species.
Cyclic voltammetry measurements conducted on previously
reported (i-Pr-POCNamine)NiBr complexes have shown that
these complexes undergo reversible or quasi-reversible redox
behavior (Eox1/2 ∼ 500−600 mV with respect to the ferrocene/
ferrocenium redox couple).14a CV measurements conducted on
our (Ph-POCNimine)NiBr complexes showed one-electron
oxidations at comparable potentials (see Figure S18 in the
Supporting Information), but only complex 4 showed reversible
oxidation. This might imply that the putative, electrochemically
generated trivalent species [(Ph-POCNimine-R)NiBr]+ are less
stable in comparison to [(i-Pr-POCNamine)NiBr]+. Never-
theless, we set out to investigate the preparation of stable
and isolable trivalent derivatives based on Ph-POCNimine
ligands.
Several attempts were made to oxidize complexes 1−4 using
chemical oxidants such as Br2, I2, N-bromosuccinimide (NBS),
and N-chlorosuccinimide; these oxidation attempts were
performed under reaction conditions that had proved favorable
for the previous syntheses of (i-Pr-POCN amine)NiBr214a and (i-
Pr-POCNimine-Bn)NiBr2.14c Our initial observations were
consistent with the anticipated oxidation reaction: the original
yellow/orange color of the reaction mixtures containing (Ph-
POCNimine)NiBr darkened upon addition of the oxidant, and
NMR spectroscopy indicated conversion of the diamagnetic
Ni(II) precursors to NMR-silent species. The in situ generated
products were found to be fairly soluble in dichloromethane but
less so in toluene or hexane; regardless of solvent, however, the
reaction mixtures decomposed gradually at room temperature
to give yellow (1, 3, 4) or reddish (2) solutions of new species
that were also NMR-silent.
Figure 2. ORTEP diagram for complex 5. Thermal ellipsoids are set at
the 50% probability level. Most hydrogen atoms are omitted for clarity.
Selected bond distances (Å) and angles (deg): Ni1−Br1 = 2.3702(5);
Ni1−Br2 = 2.4113(6); Ni1−Br3 = 2.3591(7); Ni1−O2 = 1.963(2);
P1−O1 = 1.598(2); P1−O2 = 1.481(2); N1−C7 = 1.295(4); N1−C8
= 1.434(3); P1−C14 = 1.792(2); P1−C20 = 1.776(2); Br1−Ni1−Br2
= 101.81(2); Br1−Ni1−Br3 = 128.82(2); Br2−Ni1−Br3 = 111.01(2);
Br1−Ni1−O2 = 103.66(7); Br2−Ni1−O2 = 103.26(7); Br3−Ni1−
O2 = 105.55(7); O1−P1−O2 = 114.1(1); C7−N1−C8 = 125.3(2);
C6−C7−N1 = 124.8(2).
oxidation by addition of a Br atom to Ni, the observed
transformation of 2 consists of a two-electron oxidation of the
phosphinite moiety and a net addition of two HBr to the Ni−C
and Ni−N moieties. We speculate that this unusual trans-
formation is the result of a hydrolytic decomposition of the
putative, thermally unstable trivalent intermediate.
The structural parameters for 5 (Figure 2) confirm that P−
O2 < P−O1 (ca. 1.48 vs 1.60 Å) and N−C7 < N−C8 (ca. 1.30
vs 1.43 Å). The Ni center adopts a distorted tetrahedral
geometry with fairly uniform Br−Ni−O angles (103−106°) but
quite different Br−Ni−Br angles (101−129°); one of the Ni−
Br bonds is also significantly longer than the others (2.41 vs
2.36−2.37 Å). Comparison of these parameters to literature
values for related species indicates that the observed nonuni-
form Ni−Br distances and Br−Ni−Br angles might be due to
the proximity of the NiBr3 moiety to the positively charged
fragment inside the unit cell.25
Next, we undertook an oxidation experiment at low
temperature in an attempt to slow down and arrest the
putative trivalent species. Complex 2 was treated with Br2 in
CH2Cl2 at −78 °C, and the resulting dark reaction mixture was
kept at ca. −75 °C to give a yellowish precipitate over a period
of days. Closer inspection of this precipitate showed that it
consisted of an amorphous, NMR-silent yellow solid as well as
small quantities of NMR-silent green crystals. X-ray diffraction
analysis conducted on the latter identified it as [Br3NiII{κO-(E)-
N- ( 3 - ( ( di p h e ny l ph o s p ho r y l ) o x y ) b e nz yl i d e ne ) -
The failure to isolate the target trivalent species (Ph-
POCNimine-R)NiX2 prompted us to monitor the oxidation
reaction and measure thermal stabilites of the products by UV−
vis spectroscopy, as had been done previously for the fully
characterized trivalent complex (i-Pr-POCNimine-Bn)NiBr2.14a
We found that addition of 0.6 equiv of Br2 to CH2Cl2, CCl4,
D
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Figure 3. UV−vis spectra of nickel complexes before and after addition of 3 equiv of Br2: (a) 1 (10−3 M in DCM) at 25 °C; (b) 2 (10−3 M in
toluene) at 70 °C).
Scheme 3. Synthesis of Complexes 6 and 7
acetonitrile, toluene, or THF solutions of the bromo derivatives
1−4 caused an immediate and significant increase in the
intensities of the absorptions at ca. 400 nm, and weak-intensity
absorptions emerged in the 480−520 nm region (Figure 3);
these observations are comparable to what has been observed
previously during the oxidation of (i-Pr-POCNimine-Bn)NiBr.
For the latter complex, it had been noted that slow
decomposition of the oxidized species led to regeneration of
the initial divalent species, as confirmed by monitoring the time
profile of the UV−vis spectra.14c In contrast, the UV−vis
spectra of the putative (Ph-POCNimine-R)NiBr2 complexes
studied here indicated a much faster decomposition process
that did not regenerate the initial divalent precursors via a
simple unimolecular dissociation of a bromine radical. We
conclude, therefore, that the chemical oxidation of (Ph-
POCNimine-R)NiBr is not reversible as the thermally unstable
trivalent species undergo ligand degradation (vide infra).
Altogether, the above observations indicated that trivalent
species supported by Ph-POCNimine might be inherently too
unstable to be isolated. In order to confirm the crucial
importance of P-substituents for the thermal stability of
trivalent species based on POCNimine ligands, we set out to
prepare the i-Pr2P-analogue of 2 and attempt its oxidation, as
described below.
POCNimine-Bn)NiBr2, 7, as a black paramagnetic solid in 63%
yield; characterization of this compound will be described
below. The successful preparation and isolation of 7 allowed us
to conclude that the choice of P-substituent is very crucial for
stabilizing Ni(III) species. This finding also encouraged us to
further probe the stabilities of POCNimine-based trivalent
derivatives as a function of other factors, including the nature
of the anionic ligands X. The latter was suspected to be an
important factor because our previous studies on PCP-,
POCOP-, and POCN-based Ni(III) complexes had indicated
that the dibromo derivatives are by far more stable than their
dichloro and diiodo analogues; moreover, mixed-halogen
species have also proven to be unstable to isolation. Indeed,
nearly all known examples of such complexes feature identical
X ligands, i.e., (pincer)NiX2 or (pincer)NiX2(L) (X = halide or
NCS), the only confirmed example of a “heteroleptic”
(pincer)Ni(III) species being (NCN)NiBr(Cl), reported in
2009 by Kozhanov et al.20d,26 It appeared, therefore, that
“heteroleptic” complexes of the type (pincer)NiX(Y) might be
inherently unstable. In an effort to further investigate this
phenomenon with POCNimine-type ligands, we carried out the
experiments described below to test the feasibility of isolating
trivalent, 17-electron (POCNimine)NiXY.
In the first experiment, the bromo derivative 6 was treated
with N-chlorosuccinimide with the objective of preparing (i-Pr-
POCNimine-Ph)NiIII(Br)Cl. In contrast to the relatively
straightforward preparation and isolation of 7, we found that
oxidation of 6 with N-chlorosuccinimide proved much more
complicated. Evaporation of the dark-brown solution obtained
from this reaction gave solid residues, which were washed with
hexane and recrystallized from CH2Cl2 to give a deep-brown
powder containing some red crystals. The latter were physically
separated from the bulk material, and both components were
subjected to recrystallization attempts. These attempts having
proven unsuccessful, we undertook a column chromatography
of the mixture (80/20, hexane/DCM) and obtained two
reddish bands, which were characterized as follows.
Preparation and Chemical Oxidation of (i-Pr-POC-
imine-Ph)NiBr, 6. Compound 6 was prepared in a
N
straightforward, one-pot procedure as follows: a THF mixture
of the preligand b, ClP(i-Pr)2, and NEt3 was refluxed for 2 h,
and then evaporated to dryness; to the residue was added a
toluene suspension of NiBr2(MeCN)n and NEt3, and the
resulting mixture was refluxed for 2 h (Scheme 3). The target
product 6 was obtained in 75% yield after standard workup and
chromatography, and fully characterized by elemental analysis,
NMR spectroscopy, and X-ray crystallography (vide infra).
With the new complex 6 in hand, we attempted its oxidation
by treatment with Br2 or NBS, which proved successful and
allowed us to isolate the target trivalent species (i-Pr-
E
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Scheme 4. Synthesis of Complex 9 and Attempted Oxidation of Complexes 6 and 9
Figure 4. ORTEP diagrams for complexes 6 (a), 7 (b), and 9 (c). Thermal ellipsoids are set at the 50% probability level. Hydrogen atoms are
omitted for clarity. A selection of structural parameters is listed in Table 2.
The first band consisted of a diamagnetic compound showing
ppm) and the 13C doublet for the imine carbon (174 vs 172
ppm; JP−C = 2−3 Hz). One notable difference is the chemical
3
a
31P singlet resonance similar but not identical to that of the
starting material 6 (200.6 ppm vs 201.2 ppm). This band
yielded small quantities of reddish crystals that appeared to be a
new derivative of 6 in which the N-Ph moiety had been
brominated at the para position; unfortunately, however, an
unequivocal assignment was not possible owing to the low
quality of the data obtained.27,28 The second band consisted of
an NMR-silent compound that could be identified unequiv-
ocally as the trinuclear complex 8 featuring Ni(II) units bridged
by chloride and succinimide ligands (Scheme 4), to be
discussed below.
shift of the imine proton, which resonates at 8.43 ppm in 6 (d,
4JHP = 4) compared to ca. 7.3 ppm in its N-Bn analogue. The
structural analysis of a single crystal of 6 (Figure 4a, Table 2)
allowed us to compare its solid state structure to that of its
PPh2 analogue 2. The overall structures of these complexes are
quite similar, as are most of the analogous distances. For
instance, the Ni−P distance is insignificantly shorter in 6
(2.1067(8) vs 2.1099(5) Å) and the Ni−N distances are nearly
equivalent (2.003(2) vs 1.9994(1) Å). These observations
imply comparable trans influences for i-Pr2P and Ph2P moieties.
X-ray diffraction analysis of the trivalent complex 7 revealed a
pentacoordinated geometry (Figure 4b), which is best
described as distorted square pyramidal based on the small
value of the τ parameter calculated for this structure (ca.
0.023).31 This is comparable to the τ values observed for
analogous trivalent complexes (i-Pr-POCNamine)NiBr2 (ca.
0.032),14a (i-Pr-POCNimine-Bn)NiBr2 (ca. 0.05),14c
(POCsp3OP)NiBr2 (ca. 0.06),3i,j and (PCsp3P)NiBr2 (ca.
0.028).3k
Inspection of the structural parameters in 7 (Table 2) reveals
a number of features and deformations commonly found in
pentacoordinated complexes of trivalent nickel.3i−k,14a,c For
instance, the phosphinito moiety is slightly outside of the plane
defined by the atoms C, Ni, and N and the aromatic ring (0.41
Å); a pyramidal distortion is also evident from (a) the out-of-
plane displacement (by ca. 0.32 Å) of the Ni atom in 7 from the
basal plane defined by the atoms P1, C1, N1, and Br1 and (b)
the C(1)−Ni−Br(1) angle that is much smaller in 7 than 6
(159° vs 174°). The axial Ni−Br bond is also much longer than
its equatorial counterpart: 2.45 vs 2.36 Å; the latter observation
The above-described unsuccessful attempt to prepare a
“heteroleptic” trivalent species (i-Pr-POCNimine-Ph)NiIII(Br)X
by oxidation of the bromo derivative prompted us to attempt
oxidation of another divalent derivative (i-Pr-POCNimine
-
Ph)NiII(X) with NBS, which in our hands had proven to be
the most reliable oxidant for the preparation of trivalent species.
We selected the thiocyanate derivative (X = NCS) as a suitable
candidate for this purpose because of the anticipated ease in its
characterization by X-ray crystallography.29 Thus, the Ni-NCS
derivative of 6 was prepared by treating it with KNCS in
dichloromethane (Scheme 4); the complete characterization of
the new derivative 9 will be described below. Unfortunately,
oxidation of the latter proved unsuccessful: treatment of 9 with
N-bromosuccinimide led to regeneration of 6 and formation of
a side product that was identified (by X-ray diffraction studies)
as the known compound N-thiocyanatosuccinimide30 (Scheme
4).
Characterization of Complexes 6−9. The NMR features
of complex 6 are fairly similar to those of its N-Bn analogue
reported previously,14c including the 31P singlet (200 vs 201
F
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most striking feature of this structure is the tetradentate ligand
formed from an unexpected “fusing” of three succinimide
moieties. The orthoamide-like character of this ligand is defined
by the quaternary carbon C1 that has presumably formed via a
condensation reaction between a succinimide fragment and two
succinimide NH moieties. This in situ generated ligand binds
the peripheral Ni atoms in a tripodal κO,κN,κO fashion, while
one carbonyl moiety of a tethered succinimide group serves as a
bridge to the central Ni atom. Two bridging chlorides and the
carbonyl moiety of an independent succinimide ligand
complete the octahedral coordination sphere of the peripheral
Ni(II) centers. The bite angle of the bridging succinimide
moiety causes the central and peripheral coordination planes to
be tilted with respect to each other; the hinge angle defined by
the planes Cl1−Ni1−Cl2 and Cl1−Ni2−Cl2 is about 37°.
The Ni(II)−Ni(II) distance of 3.24 Å found in 8 is
significantly longer than the sum of two Ni(II) covalent radii
(2.5 Å), yet much shorter than in analogous multinuclear
compounds featuring a Ni(II)(μ-Cl)2Ni(II) fragment;32 this is
likely due to the constraints of the tetradentate ligand. The
shortest Ni−O distance (ca. 2.06 Å) is formed between the
central Ni atom and the oxygen of the bridging carbonyl
moiety, within the 5-membered chelating ring. Longer and
significantly unequal Ni−O distances are found in the two 6-
membered chelate rings formed by the tripodal ligand and the
peripheral Ni atoms, Ni1−O1−2.06 Å and Ni1−O2−2.12 Å,
whereas the monodentate succinimide O atom is at an
intermediate distance from the peripheral Ni center (2.08 Å).
Complex 9 (Figure 4c) is isostructural to complexes 2−4 and
6. Comparison of the structural parameters for complexes 6 and
9 (Table 2) reveals that the Ni−C distances are very similar,
implying similar trans influences for Br and NCS. On the other
hand, replacing Br by NCS results in a slightly longer Ni−P
distance, a slightly shorter Ni−N distance, greater C−Ni−X
and P−Ni−X angles, and a smaller N−Ni−X angle; all of these
observations can be attributed to the shorter Ni−NCS distance
in 9 relative to the Ni−Br distance in 6 (ca. 1.88 vs 2.33 Å).33
Table 2. Selected Bond Distances (Å) and Angles (deg) for
Complexes 6, 7, and 9
(i-Pr-POCNimine
-
14c
parameter
6
7
9
Ph)NiBr2
Ni−C(1)
Ni−P(1)
Ni−N1
Ni−X(1)
Ni−X(2)
N−Cimine
1.851(3)
2.1067(8)
2.003(2)
2.3331(5)
1.902(4)
2.198(1)
2.012(4)
2.3570(9)
2.4467(9)
1.288(6)
78.8(1)
1.849(2)
2.1216(9)
1.982(2)
1.879(2)
1.896(2)
2.1919(5)
2.004(2)
2.3597(4)
2.4191(3)
1.285(3)
78.43(6)
1.298(3)
80.98(9)
1.292(3)
80.41(7)
C(1)−Ni−
P(1)
C(1)−Ni−
82.6(1)
81.8(2)
82.84(9)
172.7(1)
82.52(7)
155.43(5)
95.92(5)
158.25(5)
100.17(4)
93.14(4)
92.99(2)
99.13(2)
108
N(1)
C(1)−Ni−
173.75(9)
159.1(1)
97.3(1)
X(1)
C(1)−Ni−
Br(2)
N(1)−Ni−
163.58(6)
103.55(6)
157.7(1)
100.0(1)
94.8(1)
163.21(6)
98.77(9)
P(1)
N(1)−Ni−
X(1)
N(1)−Ni−
X(2)
P(1)−Ni−
92.85(2)
94.39(4)
98.42(4)
103.3(3)
97.99(7)
X(1)
P(1)−Ni−
X(2)
X(1)−Ni−
X(2)
2
can be attributed to the partially filled dz orbital and steric
repulsions between the P-substituents and the lone pairs of the
axial bromine atom. As expected, most Ni−ligand bond
distances are somewhat longer in 7 vs 6 due to the greater
coordination number of the Ni center in 7.
Complex 8 (Figure 5, Table 3) is a trinickel species featuring
distorted octahedral units and a central Ni atom that is located
on a center of reflection (space group = R3). The cis and trans
̅
angles range from 84° to 95° and 170° to 180°. Perhaps the
Figure 5. ORTEP diagram for complex 8. Thermal ellipsoids are set at the 50% probability level. Hydrogen atoms are omitted for clarity, as are
severely disordered molecules of CH2Cl2 and one-half of a hexane molecule. Selected bond distances and angles are listed in Table 3.
G
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Table 3. Selected Bond Distances (Å) and Angles (deg) for Complex 8
distances
angles
Ni(1)−Ni(2)
Ni(1)−O(1)
Ni(1)−O(3)
Ni(1)−O(5A)
Ni(1)−N(4)
Ni(1)−Cl(1A)
Ni(1)−Cl(2)
Ni(1)−Cl(2A)
Ni(2)−Cl(1)
Ni(2)−O(7)
3.237(1)
2.055(3)
2.116(4)
2.09(2)
Cl(1)−Ni(2)−O(7)
Cl(2A)−Ni(2)−O(7A)
Cl(2)−Ni(2)−Cl(1)
Cl(1A)−Ni(1)−Cl(2)
Cl(1A)−Ni(1)−O(1)
Cl(1A)−Ni(1)−O(3)
Cl(1A)−Ni(1)−O(5A)
Cl(1A)−Ni(1)−N(4)
Cl(2)−Ni(1)−O(1)
Cl(2)−Ni(1)−O(3)
85.4(1)
91.0(1)
87.98(4)
90.27(5)
90.5(1)
174.7(1)
90.8(5)
94.3(1)
177.9(1)
94.9(1)
Cl(2)−Ni(1)−O(5A)
Cl(2)−Ni(1)−N(4)
O(1)−Ni(1)−O(3)
O(1)−Ni(1)−O(5A)
O(1)−Ni(1)−N(4)
O(3)−Ni(1)−O(5A)
O(3)−Ni(1)−N(4)
O(5A)−Ni(1)−N(4)
Ni(1)−Cl(1A)−Ni(2)
Ni(2)−Cl(2A)−Ni(1)
94.3(5)
94.5(1)
84.3(1)
83.8(5)
87.4(2)
87.3(5)
86.9(1)
170.9(8)
85.34(4)
84.53(4)
1.983(4)
2.359(1)
2.386(1)
2.427(1)
2.416(1)
2.042(3)
anticipated Schiff base was obtained as a brown suspension. Removal
of the volatiles under vacuum for 1 h gave b as a brown powder (1.55
g, 96%). 1H NMR (CD3OD, 400 MHz, 298 K): δ 8.34 (s, 1H, −CH
N), 7.4−6.9 (m, 9H, −H-Ar), 5.1 (s, 1H, −OH, overlapping with
MeOD). 13C {1H} NMR (δ CD3OD, 400 MHz, 298 K): 163 (s, 1C,
−CHN), 159.1 (s, 1C, {Ar}C1), 152.6 (s, 1C, {Ar}C8), 138.4 (s, 1C,
{Ar}C5), 130.9 (s, 1C, {Ar}C3), 130.2 (s, 2C, {Ar}C10and12), 127.2 (s,
1C, {Ar}C11), 121.9(s, 2C, {Ar}C9and13), 121.9(s, 1C, {Ar}C4),120 (s,
1C, {Ar}C4), 119.3 (s, 1C, {Ar}C2), 115.2 (s, 1C,{Ar}C6). Anal. Calcd
for C13H11NO: C, 79.16; H, 5.62; N, 7.10. Found: C, 79.16; H, 5.62;
N, 7.08.
3-(tert-Butyliminomethyl)phenol (c). To a solution of 3-
hydroxybenzaldehyde (1.0 g, 8.29 mmol) in 7 mL of ethanol at rt
was added a solution of terbutylamine (0.66 g, 9.04 mmol)in 5 mL of
ethanol. The resulting mixture was stirred for 2.5 h while the in situ
generated water was being collected in a Dean−Stark trap. The
anticipated Schiff base was obtained as a yellow suspension. Removal
of the volatiles under vacuum gave c as a yellowish powder, (1.2 g,
82.5%). 1H NMR (CD3OD, 400 MHz, 298 K): δ 8.26 (s, 1H, −CH
N), 7.20 (s, 3H, ArH3,4,5), 6.88 (s, 1H, ArH2), 1.30 (s, 9H, CH3−C),
5.05 (s, 1H, OH overlapping with MeOD). 13C {1H} NMR (δ
CD3OD, 400 MHz, 298 K): 158.8 (s, 1C, −C−OH), 139.1 (s, 1C,
{Ar}C5), 130.6 (s, 1C, {Ar}C3), 120.9 (s, 1C, {Ar}C4), 118.8 (s, 1C,
{Ar}C2), 115.2 (s, 1C, {Ar}C6), 58 (s, 1C,{Ar}C8), 30 (s, 3C (CH3)3);
the −CHN signal was obscured in this spectrum but could be
identified at 154.82 ppm from the 2D spectrum (Figure S1 in the
Supporting Information). Anal. Calcd for C11H15NO: C,74.54; H,
8.53; N, 7.90. Found: C, 73.11; H, 8.48; N, 7.42.
CONCLUSIONS
■
The study reported herein has allowed a comparison of spectral
and solid state features of (POCNimine)NiX complexes as a
function of P- and N-substituents. Electrochemical measure-
ments indicated that the one-electron oxidation of these
complexes is fairly insensitive to the heteroatom substituents,
but studies of chemical oxidation showed that divalent
precursors featuring the OP(i-Pr)2 donor moiety led to
trivalent derivatives that are isolable and thermally stable. In
contrast, oxidation of their OPPh2 counterparts led to
decomposition and a mixture of side products, one of which
was identified as the zwitterionic complex 5. Stabilities of
trivalent compounds (i-Pr-POCNimine)NiX2 were also found to
be sensitive to the nature of oxidation agent, NBS being
superior to its chloro analogue, and nature of X ligand (Br2 >
Br(Cl), Br(NCS)). Current studies are aimed at probing the
electron transfer reactivities of (i-Pr-POCNimine-R)NiBr2 as a
function of N-substituents R.
EXPERIMENTAL SECTION
■
General. Unless otherwise stated, all manipulations were carried
out using standard Schlenk and glovebox techniques under nitrogen
atmosphere. All solvents used for experiments were dried to water
contents of less than 10 ppm by passage through activated aluminum
oxide (MBraun SPS) and deoxygenated by three vacuum−nitrogen
purge cycles. Quality of the solvents used for experiments was tested
using a Mettler Toledo C20 coulometric Karl Fischer titrator.
Compound a, 3-((benzylimino)methyl)phenol, was prepared accord-
ing to a previously reported procedure.14c The following were
purchased from Aldrich and used without further purification: 3-
hydroxybenzaldehyde, benzylamine, aniline, tert-butylamine, cyclo-
hexanamine, chlorodiphenylphosphine, chlorodiisopropylphosphine,
Ni (metal), N-chlorosuccinimide, N-bromosuccinimide, bromine,
iodine, acetone-d6, C6D6, CD3OD, and CD2Cl2. NEt3 was distilled
and kept over 4 Å molecular sieves. Bruker AV 400 and AV 500
[3-((Cyclohexylimino)methyl)phenol] (d). To a solution of 3-
hydroxybenzaldehyde (1.0 g, 8.2 mmol) in 10 mL of ethanol at rt was
added a solution of cyclohexylamine (0.9 g, 9.02 mmol) in 5 mL of
ethanol. The resulting mixture was stirred for 3 h while the in situ
generated water was being collected in a Dean−Stark trap. The
anticipated Schiff base was obtained as a beige suspension. Removal of
the volatiles under vacuum gave d as a beige powder, (1.1 g, 69%). 1H
NMR (CD3OD, 400 MHz, 298 K): δ 8.24 (s, 1H, −CHN), 7.24−
6.91 (m, 3H, {Ar}H3,4,6), 6.89−6.87 (d, 1H, {Ar}H2), 5.10 (s, 1H,
OH), 3.17 (br m, 1H, CyH8), 1.82−1.22 (m, 10H, -CyH9,10,11,12,13).
13C NMR (CD3OD, 400 MHz, 298 K): δ 162.2 (s, 1C, NCH−),
spectrometers were used for recording H, 13C (101 MHz), and 31P
1
(162 MHz) NMR spectra. H and 13C chemical shifts are reported in
1
159 (s, 1C, ArC1), 138.5 (s, 1C, ArC5), 130.7 (s, 1C, ArC3), 121 (s,
1C, ArC4), 119.1 (s, 1C, ArC2), 115.4 (s, 1C, ArC6), 71.1 (s, 1C,
CyC8), 35.2 (s, 2C, CyCortho), 26.6 (s, 1C, CyCpara), 25.8 (s, 1C,
CyCmeta). Anal. Calcd for C13H17NO: C, 76.81; H, 8.43; N, 6.89.
Found: C, 77.31; H, 9.34; N, 4.74.
ppm downfield of TMS and referenced against the residual proton
signals for acetone-d6 (2.05 ppm for H and 206.26 ppm for 13C),
1
CD3OD (3.31 ppm for 1H and 49.00 ppm for 13C), and CD2Cl2 (5.32
1
ppm for H and 53.84 ppm for 13C). 31P NMR chemical shifts are
reported in ppm and referenced against the signal for 85% H3PO4
(external standard, 0 ppm). Coupling constants are reported in Hz.
The correlation and assignment of 1H and 13C NMR resonances were
aided by 1H COSY, HSQC, and HMQC. UV−vis spectra were
recorded on Varian Bio 300 equipped with a temperature controlling
system using standard sampling cells (1 cm optical path length).
3-((Phenylimino)methyl)phenol (b). To a solution of 3-
hydroxybenzaldehyde (1.00 g, 8.19 mmol) in 5 mL of ethanol at rt
was added a solution of aniline (0.84 g, 9.01 mmol) in 5 mL of
ethanol. The resulting mixture was stirred for 3 h while the in situ
generated water was being collected in a Dean−Stark trap. The
(3-((Diphenylphosphino)oxy)benzylidene)-1-phenylme-
thanamine (e). To a solution of ClPPh2 (2.45 mL, 11.12 mmol) in
THF (10 mL) was added a solution of a (1.56 g, 7.41 mmol) and NEt3
(1.34 mL, 9.63 mmol) in THF (10 mL). The resulting mixture was
stirred for 3 h. Evaporation of the solvent and extraction of the residue
with toluene (4 × 10 mL) followed by filtration of the combined
extracts and evaporation gave the desired product e as a yellowish oil,
(2.40 g, 82%).
1H NMR (CDCl3, 500 MHz, 298 K): δ 8.26 (br s, 1H, CHN),
7.5−7.4 (m, 6H, {Ph2P}Hortho and {Ph2P}Hpara), 7.37−7.30 (m, 8H,
{Ph2P}Hmeta, {Bn}Hortho and {Bn}Hmeta), 7.28 (m, 1H, {Bn}Hpara),
H
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3
7.24−7.18 (m, 2H, {Ar}Hmeta), 7.14−7.12 (m, 1H, {Ar}Hpara), 4.74
(br s, 2H, {Bn}CH2). 13C{1H} NMR (δ, CDCl3, 125 MHz, 298 K): 65
(s, 1C, {Bn}CH2), 118.54−114.46 (d, J = 10, 1C, {Ar}C3), 121.25−
121.15 (d, J = 12, 1C, {Ar}C5), 122.8 (br s, 1C, {Ar}C4, 127 (s, 1C,
{Bn}Cpara), 128 (br s, 2C, {Bn}Cortho), 128.7 (s, 2C, {Bn}Cmeta), 128.6
(br s, 4C, m-C in PPh2), 130 (s, 2C, p-C in PPh2), 130.8−130.6 (d, JCP
= 22 Hz, 4C, o-C in PPh2), 132−131.8 (d, JCP = 24 Hz, 2C, i-C in
PPh2), 138 (s, 1C, {Bn}Cipso), 139.3 (s, 1C, {Ar}C6), 140.98−140.75
m-C in PPh2) 130.74−130.57(d, JCP = 22.3 Hz, 4C, o-C in PPh2),
135.56−135.34 (dd, JCP = 19.25 Hz 2C, i-C in PPh2), 138.45 (s, 1C,
{Ar}Cpara), 140.95−140.82 (d, JCP = 17.5 Hz, 1C, {Ar}C1), 157.73−
2
157.65 (m, 1C, {Ar}C6), 158.2 (br s, 1C, CHN). 31P{1H} NMR
(CDCl3, 202 MHz, 298 K): δ 110 (s, 1P). MS (ESI-HRMS, hexane)
(m/z): [M + H]+ (C25H27NOP) calcd 388.18248; found 388.18228;
diff 0.51 ppm.
κP,κC,κN-{2,6-(Ph2PO)(C6H3)(CHNBn)}NiBr (1). A solution of e
(0.30 g, 0.76 mmol) in 2 mL of toluene was slowly added to the
stirring suspension of NiBr2(CH3CN)x (0.34 g, 1.14 mmol) and NEt3
(0.16 mL, 1.14 mmol) in toluene (5 mL) at rt. The resulting dark
brown mixture was then heated for 2 h at 110 °C. Removal of the
solvent and purification of the solid residues by column chromatog-
raphy on SiO2 (eluents: 50:50 CH2Cl2:hexane) furnished 1 as an
(d, J = 17 Hz, 1C, {Ar}C1), 157.82−157.73 (d, J = 10 Hz, 1C,
{Ar}C2), 161.6 (br s, 1C, CHN). 31P{1H} NMR (CDCl3, 202 MHz,
298 K): δ 111 (s, 1P). MS (ESI-HRMS, hexane) (m/z): [M + H]+
(C26H23NOP) calcd 396.15118; found 396.15065; diff 1.33 ppm.
(3-((Diphenylphosphino)oxy)benzylidene)aniline (f). To a
solution of ClPPh2 (2.13 mL, 9.64 mmol) in THF (5 mL) was
added a solution of b (1.46 g, 7.42 mmol) and NEt3 (1.55 mL, 11.12
mmol) in THF (15 mL). The resulting mixture was stirred for 2 h.
Evaporation of the solvent and extraction of the residue with toluene
(4 × 10 mL) followed by filtration of the combined extracts and
evaporation gave the desired product f as a brown oil, (2.32 g, 82%).
1H NMR (CDCl3, 500 MHz, 298 K): δ 8.4 (br s, 1H, CHN),
7.7−7.26 (m, 13H, {Ph2P}Hortho, {PhN}Hpara, {Ph2P}Hpara, {Ph2P}
2
2
1
orange solid (0.36 g, 88%). H NMR (CD3CN, 500 MHz, 298 K): δ
8.25−8.24 (d of ps-t, 4JHP = 3.8 Hz, 1H, CHN), 7.97−7.93 (m, 4H,
{Ph2P}Hortho), 7.64−7.60 (m, 2H, {Ph2P}Hpara), 7.56−7.52 (m, 4H
{Ph2P}Hmeta), 7.46−7.44 (m, 2H, {Bn}Hortho), 7.39−7.36 (m, 2H,
{Bn}Hmeta), 7.32−7.29 (m, 1H, {Bn}Hpara), 7.12−7.06 (m, 2H,
{Ar}Hmeta), 6.84−6.82 (dd, JHH = 1.4 Hz, 1H, {Ar}Hpara), 4.94 (br s,
2H, {Bn}CH2). 13C{1H} NMR (δ, (CD3)2CO, 125 MHz, 298 K): 60
(s, 1C, {Bn}CH2), 114.89−114.77 (d, J = 14, 1C, {Ar}C3), 122.8 (br s,
1C, {Ar}C5), 128.3 (br s, 2C, {Ar}C4 and {Bn}Cpara), 128.4 (br s, 2C,
H
meta, {PhN}Hortho), 7.25−7.21 (m, 2H, {PhN}Hmeta), 7.20−7.17 (m,
3H, {Ar}Hmeta and {Ar}Hpara). 13C{1H} NMR (δ, CDCl3, 125 MHz,
{Bn}Cortho), 130 (s, 2C, {Bn}Cmeta), 129.68−129.48 (t, JCP = 13 Hz,
3
298 K): 118.84−118.76 (d, JCP = 11 Hz, 1C, {Ar}C5), 126 (s, 1C,
3
{Ar}C3), 128 (br s, 2C, {Ar}Cpara and {PhN}Cpara), 129 (br s, 2C,
{PhN}Cortho), 130 (s, 2C, {PhN}Cmeta), 130.8−130.6 (d, JCP = 23 Hz,
4C, o-C in PPh2), 132.8−131.4 (m, 6C, m-C in PPh2 and p-C in
PPh2), 135.6−135.4 (m, 1C, {Ar}C2), 138 (s, 1C, { PhN }Cipso),
140.81−140.67 (d, JCP = 18 Hz, 2C, i-C in PPh2), 152 (s, 1C,{Ar}C1),
4C, m-C in PPh2), 133.07−132.76 (m, 6C, o-C in PPh2 and p-C in
PPh2), 133.64−133.33 (d, JCP = 39 Hz, 2C, i-C in PPh2), 139.11 (s,
1C, {Bn}Cipso), 149.2 (s, 1C, {Ar}C6), 153.54−153.24 (d, 2J = 37 Hz,
1C, {Ar}C1), 164.39−164.28 (d, 2J = 13 Hz, 1C, {Ar}C2), 175.7 (br s,
1C, CHN). 31P{1H} NMR ((CD3)2CO, 202 MHz, 298 K): δ 150
(s, 1P). Anal. Calcd for [C26H21NBrNiOP]: C, 58.59; H, 3.97; N, 2.63.
Found: C, 59.55; H, 3.98; N, 2.52.
157.95−157.87 (d, J = 10 Hz, 1C, {Ar}C6), 160 (br s, 1C, CHN).
2
31P{1H} NMR (CDCl3, 202 MHz, 298 K): δ 112 (s, 1P). MS (ESI-
HRMS, hexane) (m/z): [M + H]+ (C25H21NOP) calcd 382.13553;
found 382.13516; diff 0.96 ppm.
κP,κC,κN-{2,6-(Ph2PO)(C6H3)(CHNPh)}NiBr (2). A solution of f
(2.32 g, 6.08 mmol) in 5 mL of toluene was slowly added to the
stirring suspension of NiBr2(CH3CN)x (2.74 g, 9.12 mmol) and NEt3
(1.27 mL, 9.12 mmol) in toluene (5 mL) at rt. The resulting dark
brown mixture was then heated for 3 h at 110 °C. Removal of the
solvent and purification of the solid residues by column chromatog-
raphy on SiO2 (eluents: 30:70 CH2Cl2:hexane) furnished complex 2 as
(3-((Diphenylphosphino)oxy)benzylidene)-tert-butyl (g). To
a solution of c (0.9 g, 5.09 mmol) and NEt3 (1.06 mL, 7.63 mmol) in
THF (15 mL) was added ClPPh2 (1.13 mL, 6.1 mmol). The resulting
mixture was stirred at 60 °C for 3 h and evaporated under pressure to
give an oily heterogeneous mixture. The desired product was extracted
from the salt with toluene (5 × 10 mL). Evaporation of the combined
extracts furnished g as a yellowish oil (m = 1.72 g, 94%). 31P{1H}
NMR (CD2Cl2, 202 MHz, 298 K): δ 112 (s, 1P).
1
a brick-red powder (2.40 g, 76%). H NMR ((CD3)2CO, 500 MHz,
4
298 K): δ 8.48−8.47 (d, JHP = 4 Hz, 1H, CHN), 8.09−8.05 (m,
4H, {Ph2P}Hortho), 7.92−7.89 (m, 1H, {PhN}Hpara), 7.66−7.62 (m,
2H, {Ph2P}Hpara), 7.58−7.55 (m, 4H {Ph2P}Hmeta), 7.51−7.37 (m,
2H, {PhN}Hortho), 7.30−7.27 (m, 2H, {PhN}Hmeta), 7.20−7.10 (m,
1H NMR (CDCl3, 500 MHz, 298 K): δ 8.01 (br s, 1H, CHN),
7.81−7.5 (m, 8H, {Ph2P}Hortho and {Ph2P}Hmeta), 7.4−7.3 (m, 2H,
{Ph2P}Hpara), 7.17−7.12 (m, 2H, {Ar}Hmeta) 7.03 (m, 1H,{Ar}Hpara),
1.16 (s, 9H, (CH3)3−CN). 13C{1H} NMR (δ, CDCl3, 125 MHz,
298 K): 29.8 (s, 3C, (CH3)3−CN), 57 (s, 1C, (CH3)3−CN), 118
and 120 (m, 2C, {Ar}Cmeta), 122 (br s, 1C, {Ar}C2), 128.66−128.60
(d, JCP = 7 Hz, 4C, m-C in PPh2) 129.9−129.7 (m, 2C, p-C in PPh2),
2H, {Ar}Hmeta), 6.96−6.94 (dd, JHP = 7.5 Hz, 1H, {Ar}Hpara).
4
13C{1H} NMR (δ, (CD3)2CO, 125 MHz, 298 K): 115.42−115.31 (d,
3JCP = 14 Hz, 1C, {Ar}C5), 121.9 (s, 1C, {Ar}C3), 124 (br s, 2C, {Ar}
Cpara and {PhN}Cpara), 125.1 (br s, 2C, {PhN}Cortho), 130 (s, 2C,
{PhN}Cmeta), 128.49−127.91 (d, JCP = 72 Hz, 4C, o-C in PPh2),
129.8−129.5 (m, 6C, m-C in PPh2 and p-C in PPh2), 133.11−133.09
130.7−130.6 (d, 3JCP = 23 Hz, 4C, o-C in PPh2), 132−131.9 (d, JCP
=
=
(d, JCP = 2 Hz, 1C, {Ar}C2), 133.32−133.28 (d, JCP = 12 Hz, 1C, {
3
3
10.5 Hz 2C, i-C in PPh2), 139 (s, 1C, {Ar}Cpara), 141−140.8 (d, 2JCP
PhN }Cipso), 149.86−149.29 (d, JCP = 72 Hz, 2C, i-C in PPh2),
2
18 Hz, 1C, {Ar}C1), 157.73−157.65 (d, J = 10 Hz, 1C, {Ar}C6),
154.8 (s, 1C, CHN). 31P{1H} NMR (CDCl3, 202 MHz, 298 K): δ
110 (s, 1P). MS (ESI-HRMS, hexane) (m/z): [M + H]+
(C23H25NOP) calcd 362.16683; found 362.16689; diff −0.17 ppm.
(3-((Diphenylphosphino)oxy)benzylidene)cyclohexylamine
(h). To a solution of d (0.9 g, 4.33 mmol) and NEt3 (1.06 mL, 7.63
mmol) in THF (15 mL) was added ClPPh2 (0.96 mL, 5.2 mmol). The
resulting mixture was stirred at 60 °C for 4 h and evaporated under
pressure to give an oily heterogeneous mixture. The desired product
was extracted from the salt with toluene (4 × 10 mL). Evaporation of
the combined extracts furnished h as brown gel (m = 1.38 g, 82%).
1H NMR (CDCl3, 500 MHz, 298 K): δ 8.13 (br s, 1H, CHN),
7.49−7.46 (m, 4H, {Ph2P}Hortho), 7.41−7.7.39 (m, 2H, {Ph2P}Hpara),
7.37−7.32 (m, 4H {Ph2P}Hmeta), 7.17 and 7.1 (m, 3H, {Ar}Hmeta and
{Ar}Hpara), 7.05 and 7.03 (m, 1H, {Ar}H2) 1.7, 1.57, and 1.44 (m,
11H, {Cy}H). 13C{1H} NMR (δ, CDCl3, 125 MHz, 298 K): 24.9 (s,
2C, {Cy}Cmeta), 25.8 (s, 1C, {Cy}Cpara), 34.4 (s, 2C, {Cy}Cortho), 70
(s, 1C, {Cy}Cipso), 118.43−118.36 (d, JCP = 10 Hz, 1C, {Ar}C5),
120.66−120.56 (d, JCP = 12 Hz, 1C, {Ar}C2), 122.5 (s, 2C, p-C in
PPh2), 124.3 (s, 1C, 1C, {Ar}C3), 128.66−128.60 (d, JCP = 7 Hz, 4C,
154.44−154.13 (d, JCP = 38 Hz, 1C,{Ar}C1), 164.6−164.49 (d, J =
13 Hz, 1C, {Ar}C6), 175.7 (br d, JCP = 3 Hz, 1C, CHN). 31P{1H}
NMR ((CD3)2CO, 202 MHz, 298 K): δ 149 (s, 1P). Anal. Calcd for
[C25H19BrNNiOP]: C, 57.86; H, 3.69; N, 2.70. Found: C, 59.55; H,
4.03; N, 2.45.
2
2
κP,κC,κN-{2,6-(Ph2PO)(C6H3)(CHNtBu)}NiBr (3). A solution of g
(1.72 g, 4.77 mmol) in 5 mL of toluene was slowly added to the
stirring suspension of NiBr2(CH3CN)x (1.37 g, 6.20 mmol) and NEt3
(1 mL, 7.16 mmol) in toluene (5 mL) at rt. The resulting red mixture
was then heated for 3 h at 110 °C. Removal of the solvent and
purification of the solid residues by column chromatography on SiO2
(eluents: 30:70 CH2Cl2:hexane) furnished 3 as an orange powder
1
(1.71 g, 71.84%). H NMR ((CD3)2CO, 500 MHz, 298 K): δ 8.34−
8.33 (d, 4JHP = 4 Hz, 1H, CHN), 8.06−8.02 (m, 4H, {Ph2P}Hortho),
7.65−7.62 (m, 2H, {Ph2P}Hpara), 7.58−7.54 (m, 4H {Ph2P}Hmeta),
7.10 and 6.78 (m, 3H, {Ar}Hmeta and {Ar}Hpara), 1.56 (s, 9H, (CH3)3−
CN). 13C{1H} NMR (δ, (CD3)2CO, 125 MHz, 298 K): 30 (s, 3C,
(CH3)3−CN), 63 (s, 1C, (CH3)3−CN), 114 and 123 (m, 2C,
{Ar}Cmeta), 129.5−129.4 (d, JCP = 17 Hz, 1C, 1C, {Ar}C2), 133 (d, JCP
I
dx.doi.org/10.1021/om500529e | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
= 3.5 Hz, 4C, m-C in PPh2) 128.3 (s, 2C, p-C in PPh2), 133.66−
133.56 (d, 3JCP = 18.5 Hz, 4C, o-C in PPh2), 133.3−132.9 (d, JCP = 84
mL) and dried under vacuum to yield the desired product as black
(0.073 g, 63%).
Hz 2C, i-C in PPh2), 149.8 (s, 1C, {Ar}Cpara), 151.1−150.8 (d, 2JCP
=
The Trinuclear Complex 8. To a solution of 6 (100 mg, 0.22
mmol) in 5 mL of dry and degassed DCM at rt was added N-
chlorosuccinimide (38 mg, 0.29 mmol), and the resulting mixture was
stirred at rt for 45 min. Removal of the volatiles under vacuum gave a
mixture of black and dark-red powder (ca. 82 mg), which was washed
with cold hexane (3 × 5 mL) to remove hexane-soluble impurities and
recrystallized from DCM (2 mL) to give a black powder containing
red crystals suitable for X-ray diffraction analysis. In spite of the fairly
different solubilities of these two solids (the black solid is more soluble
in THF, DCM, and acetonitrile), we have not succeeded in isolating
analytically pure samples of these materials. Both of these solids are
NMR-silent.
2
65.5 Hz, 1C, {Ar}C1), 164.33−164.22 (d, J = 22 Hz, 1C, {Ar}C6),
169.37−169.35 (br d, JCP = 4.5 Hz, 1C, CHN). 31P{1H} NMR
((CD3)2CO, 202 MHz, 298 K): δ 141 (s, 1P). Anal. Calcd for
[C23H23BrNNiOP]: C, 55.36; H, 4.65; N, 2.81. Found: C, 56.4; H,
4.64; N, 2.49.
κP,κC,κN-{2,6-(Ph2PO)(C6H3)(CHNCy)}NiBr (4). A solution of h
(1.38 g, 3.55 mmol) in solution in 7 mL of toluene was slowly added
to the stirring suspension of NiBr2(CH3CN)x (1.39 g, 4.62 mmol) and
NEt3 (0.7 mL, 5.33 mmol) in toluene (5 mL) at rt. The resulting red
mixture was then heated for 3.5 h at 110 °C. Removal of the solvent
and purification of the solid residues by column chromatography on
SiO2 (eluents: 70:30 CH2Cl2:hexane) furnished 4 as an orange powder
(1.73 g, 93%). 1H NMR ((CD3)2CO, 500 MHz, 298 K): δ 8.34−8.33
κP,κC,κN-{2,6-(iPr2PO)(C6H3)(CHNPh)}Ni-NCS (9). A mixture of
6 (52 mg, 0.12 mmol) and KSCN (112 mg, 1.15 mmol) in 5 mL of
DCM was stirred overnight (15 h) at room temperature. Addition of
10 mL of hexane to the reaction mixture and filtration over a Celite
4
(d, JHP = 5 Hz, 1H, CHN), 8.07−8.02 (m, 4H, {Ph2P}Hortho),
7.64−7.61 (m, 2H, {Ph2P}Hpara), 7.58−7.55 (m, 4H {Ph2P}Hmeta),
7.10 and 6.82 (m, 3H, {Ar}Hmeta and {Ar}Hpara), 2.13, 1.81, 1.68, 1.38,
and 0.87 (m, 11H, {Cy}H). 13C{1H} NMR (δ, (CD3)2CO, 125 MHz,
298 K): 26.36 (s, 2C, {Cy}Cmeta), 26.42 (s, 1C, {Cy}Cpara), 34.1 (s,
2C, {Cy}Cortho), 63.54 (s, 1C, {Cy}Cipso), 114.4 (d, JCP = 14 Hz, 1C,
{Ar}C5), 122.5 (d, JCP = 1.7 Hz, 1C, {Ar}C3), 128.2 (s, 2C, p-C in
1
column furnished 9 as an orange jelly compound (33 mg, 67%). H
3
3
NMR (δ, CD2Cl2): 1.35−1.31 (dd, JHH = 7, JHP =15, 6H, 2 ×
3
3
CHCH3), 1.45−1.41 (dd, JHH = 7, JHP = 18.5, 6H, 2 × CHCH3),
2.36−2.29 (m, 2H, 2 × CHCH3), 6.63 (d, 3JHH = 8, 1H, {Ar}H5), 6.90
(d, 3JHH = 7, 1H, {Ar}H3), 7.0 (t, 3JHH = 8, 1H, {Ar}H4), 7.35 (m, 1H,
{Ph}Hpara), 7.4 (m, 4H, 2 × {Ph}Hortho and Ph}Hmeta), 7.9 (br s, 1H,
PPh2), 129.7−129.62 (d, JCP = 11 Hz, 1C, 1C, {Ar}C2), 133 (d, JCP
=
2
CHN). 13C{1H} NMR (δ, CD2Cl2): 16.92 (d, JCP = 1, 2C,
2.2 Hz, 4C, m-C in PPh2) 133.23−133.13 (d, 3JCP = 12.3 Hz, 4C, o-C
in PPh2), 133.7−132.7 (d, JCP = 106 Hz 2C, i-C in PPh2), 150.15 (s,
1C, {Ar}Cpara), 152.7−152.4 (d, 2JCP = 38.5 Hz, 1C, {Ar}C1), 164.44−
164.33 (d, 2J = 13 Hz, 1C, {Ar}C6), 171.07−171.04 (br d, JCP = 3 Hz,
1C, CHN). 31P{1H} NMR ((CD3)2CO, 202 MHz, 298 K): δ 148.5
(s, 1P). Anal. Calcd for [C25H25BrNNiOP]: C, 57.19; H, 4.80; N, 2.67.
Found: C, 57.53; H, 4.89; N, 2.59.
2
1
CHCH3), 17.6 (d, JCP = 5, 2C, CHCH3), 28.75 (d, J = 23, 2C,
CHCH3), 112.5 (d, J = 13, 1C, {Ar}C3), 121.7 (d, J = 2, {Ar}C5),
4
127.62 (s, 1C, {Ar}C4), 128.2 (s, 1C, {Ph}Cpara), 129.17 (s, 2C, 2 ×
{Ph}Cortho), 129.27 (s, 2C, 2 × {Ph}Cmeta), 136.85 (s, 1C, {Ph}Cipso),
142 (s, 1C, {Ar}C6), 149 (s, 1C, NCS), 151 (d, 2J = 35, 1C,{Ar}C1),
2
2
165 (d, J = 10, 1C, {Ar}C2), 173 (d, J = 3, 1C, CHN). 31P{1H}
NMR (δ, CD2Cl2): 199 (s, 1P). Most of the samples isolated were oily
solids that did not give dry powders even after several hours under
reduced pressure. Elemental analysis of one such sample showed it to
have significantly lower N and S but higher C and H compositions,
consistent with the presence of hydrocarbon impurities. (Anal. Calcd
for C20H23N2NiOPS: C, 55.98; H, 5.40; N, 6.53; S, 7.47. Found: C,
63.91; H, 8.28; N, 3.90; S, 4.88.) Thus, inclusion of two molecules of
hexane into the molecular formula of the complex would generate a
better agreement between calculated and experimental compositions,
in particular for C and H contents (Anal. Calcd for C20H23N2NiOPS·
2C6H14: C, 63.90; H, 8.55; N, 4.66; S; 5.33). The 1H NMR spectra of
the oily solid showed aliphatic resonances attributable to the
recrystallization solvent, hexane, whereas X-ray diffraction of a
crystalline sample showed no crystallization solvent in the unit cell.
Attempted Oxidation of Complex 9. To an orange solution of 9
(25 mg, 0.057 mmol) in 5 mL of dry and degassed DCM was added
N-bromosuccinimide (13 mg, 0.074 mmol), and the resulting mixture
was stirred at rt for 1 h before removal of the solvent under vacuum.
The remaining powder was then washed with cold hexane (3 × 5 mL)
and dried under vacuum to yield a mixture of dark red and brown
powder (19 mg). Unable to separate these solids, the product was
dissolved in 2 mL of CH2Cl2 and kept in the glovebox freezer (−35
°C) for 2 days. We obtained two distinct batches of crystals, red blocks
and brown plates, which were subjected to X-ray analysis. The brown
crystals proved to be the known compound N-thiocyanate succinimide
(NTS),30 whereas the red crystals were confirmed to be complex 6.
κP,κC,κN-{2,6-(iPr2PO)(C6H3)(CHNPh)}NiBr (6). To a solution
of chlorodiisopropylphosphine (0.90 mL, 5.64 mmol) in THF (5 mL)
was added a solution of b (1.01 g, 5.13 mmol) and NEt3 (1.72 mL,
7.70 mmol) in THF (10 mL). The resulting mixture was stirred at 60
°C for 2 h followed by the evaporation of the volatiles, and to this
residue was added slowly a suspension of NiBr2(CH3CN)x (2.01 g,
6.67 mmol) and NEt3 (1.72 mL, 7.70 mmol) in toluene (10 mL). The
resulting dark red mixture was then heated for an additional 2 h at 110
°C. Removal of the solvent and purification of the solid residues by
column chromatography on SiO2 (eluents: 20:80 CH2Cl2:hexane)
furnished 5 as a red powder (1.73 g, 75%). 1H NMR ((CD3)2CO, 500
3
MHz, 298 K): δ 1.38 (dd, JHH = 7, 3JHP =15, 6H, 2 × CHCH3), 1.49
3
3
(dd, JHH = 7, JHP =18, 6H, 2 × CHCH3), 2.52−2.45 (m, 2H, 2 ×
CHCH3), 6.79 (d, 4JHH = 8, 1H, {Ar}H5), 7.10 (t, 5JHH = 7.4, 1H, {Ar}
5
H4), 7.20 (d, JHH = 8, 1H, {Ar}H3),7.30−7.27 (m, 1H, {Ph}Hpara),
7.38−7.37 (m,4H, 2 × {Ph}Hortho), and 2 × {Ph}Hmeta), 8.43 (d, 4JHP
= 4, 1H, CHN). 13C{1H} NMR (δ, (CD3)2CO, 125 MHz, 298 K):
2
2
17.18 (d, JCP = 1.5, 2C, CHCH3) 18.56 (d, JCP = 3.6, 2C, CHCH3),
1
3
29.53 (d, JCP = 24, 2C, CHCH3), 114.39 (d, JCP = 12.41, 1C, {Ar}
C5), 123.34 (br s, {Ar}C3), 125 (s, 1C, {Ar}C4 and {Ph}Cpara), 127.8
(s, 2C, 2 × {Ph}Cortho), 129 (s, 2C, 2 × {Bn}Cmeta), 149.14 (s, 1C,
{Ph}Cipso), 149.81 (s, 1C, {Ar}C2), 153.73 (d, JCP = 35.3, 1C, {Ar}
2
C1), 166.27 (d, JCP = 10, 1C, {Ar}C6), 174.40 (d, JCP = 2.5, 1C,
CHN). 31P{1H} NMR ((CD3)2CO, 202 MHz, 298 K): δ 200 (s,
1P). Anal. Calcd for [C19H23BrNNiOP]: C, 50.60; H, 5.14; N, 3.11.
Found: C, 50.58; H, 5.10; N, 3.04.
2
3
κP,κC,κN-{2,6-(iPr2PO)(C6H3)(CHNPh)}NiBr2 (7). Method A. To
a solution of 6 (101 mg, 0.22 mmol) in 5 mL of dry and degassed
DCM at −78 °C was added a DCM solution of Br2 (3 mL, 0.22
mmol). The resulting mixture was allowed to stir to room temperature
for 5 min before removal of the solvent in vacuum. The dark brown-
black powder was then washed with cold hexane (3 × 5 mL) and dried
under vacuum to yield the desired product as black solid (0.096 mg,
84%).
Method B. To a solution of 6 (100 mg, 0.22 mmol) in 5 mL of dry
and degassed DCM at rt was added N-bromosuccinimide (51 mg, 0.29
mmol), and the resulting mixture was stirred at room temperature for
30 min before removal of the solvent under vacuum. The remaining
dark brown-black powder was then washed with cold hexane (3 × 5
ASSOCIATED CONTENT
■
S
* Supporting Information
NMR, IR, and UV−vis spectra; cyclic voltammograms; detailed
description of the procedures used for the single X-ray
diffraction studies, and tables of crystal data. This material is
available free of charge via the Internet at http://pubs.acs.org.
Complete details of the X-ray analyses for ligand f (1002783)
and complexes 2 (1001949), 3 (1001950), 4 (1001951), 5
(1001952), 6 (1001953), 7 (1001954), 8 (1002512), and 9
(1001955) have been deposited at The Cambridge Crystallo-
J
dx.doi.org/10.1021/om500529e | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
W.; Braunstein, P. Dalton Trans. 2012, 41, 636. (l) Zhang, J.; Gao, W.;
Lang, X.; Wu, Q.; Zhang, L.; Mu, Y. Dalton Trans. 2012, 41, 9639.
(5) Bergbreiter, D. E.; Osburn, P. L.; Liu, Y. S. J. Am. Chem. Soc.
1999, 121, 9531.
(6) Example of nonsymmetrical pincer systems: (a) Poverenov, E.;
Gandelman, M.; Shimon, L. J. W.; Rozenberg, H.; Ben-David, Y.;
Milstein, D. Chem.Eur. J. 2004, 10, 4673. (b) Gagliardo, M.;
Selander, N.; Mehendale, N.; van Koten, G.; Klein-Gebbink, R.; Szab,
K. Chem.Eur. J. 2008, 14, 4800. (c) Zhang, J.; Gandelman, M.;
Herrman, D.; Leitus, G.; Shimon, L. J.; Ben-David, Y.; Milstein, D.
Inorg. Chim. Acta 2006, 359, 1955. (d) Fischer, J.; Schurmann, M.;
Mehring, M.; Zachwieja, U.; Jurkschat, K. Organometallics 2006, 25,
graphic Data Centre. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, U.K.; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: zargarian.davit@umontreal.ca.
Notes
The authors declare no competing financial interest.
́
2886. (e) Debono, N.; Iglesias, M.; Sanchez, F. Adv. Synth. Catal.
2007, 349, 2470. (f) Durand, J.; Gladiali, S.; Erre, G.; Zangrando, E.;
Milani, B. Organometallics 2007, 26, 810. (g) Boronat, M.; Corma, A.;
ACKNOWLEDGMENTS
■
Gonzal
29, 134.
́ ́
ez-Arellano, C.; Iglesias, M.; Sanchez, F. Organometallics 2010,
D.Z. gratefully acknowledges financial support received from
NSERC of Canada (Discovery Grant) and FQRNT of Queb
́
ec
(7) For reports on catalytic activities of pincer complexes see:
(a) Ohff, M.; Ohff, A.; van der Boom, M. E.; Milstein, D. J. Am. Chem.
Soc. 1997, 119, 11687. (b) Miyazaki, F.; Shibasaki, M. Tetrahedron Lett.
1999, 40, 7379. (b) Jensen, C. M. Chem. Commun. 1999, 2443.
(d) Miyazaki, F.; Yamaguchi, K.; Shibasaki, M. Tetrahedron Lett. 1999,
40, 7379. (e) Haenel, M. W.; Oevers, S.; Angermund, K.; Kaska, W.
C.; Fan, H.-J.; Hall, M. B. Angew. Chem., Int. Ed. 2001, 40, 3569.
(f) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40, 3750.
(g) Singleton, J. T. Tetrahedron 2003, 59, 1837. (h) Sebelius, S.;
Olsson, V. J.; Szabo, K. J. J. Am. Chem. Soc. 2005, 127, 10478.
(i) Goldman, A. S.; Roy, A. H.; Huang, Z.; Ahuja, R.; Schinski, W.;
Brookhart, M. Science 2006, 312, 257. (j) Naghipour, A.; Sabounchei,
́
(Projet d’Equipe). B.M-S. is grateful to Direction des Relations
Internationales (Universite de Montreal) and Centre for Green
́
́
Chemistry and Catalysis for graduate fellowships and to the
American Chemical Society for the opportunity to attend the
ACS Summer School on Green Chemistry and Sustainable
Energy (2011). We also thank Prof. F. Schaper for judicious
advice on matters regarding some of the X-ray diffraction
studies described in this report.
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M
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