Copper(I) Complexes of Pyridine-Bridged Phosphaalkene-Oxazoline Pincer Ligands

Article abstract of DOI:10.1021/acs.inorgchem.6b00917

The synthesis of enantiomerically pure pyridine-bridged phosphaalkene-oxazolines ArP=C(Ph)(2,6-C₅H₃NOx) (1, Ar = Mes/Mes?, Ox = CNOCH(i-Pr)CH₂/CNOCH(CH₂Ph)CH₂) is reported. This new ligand forms a κ(P), κ²(NN) dimeric complex with copper(I) (7) that dissociates into a cationic κ³(PNN) monomeric complex upon addition of a neutral ligand {[1a·CuL]OTf (8a-e): L = PPh₃ (a), P(OPh)₃ (b), 2,6-lutidine (c), 4-DMAP (d), 1-methylimidazole (e)}. The P-Cu bond lengths in 8 are influenced by the π-accepting/σ-donating properties of L, and this can be observed by changes in the δ³¹P_P=C NMR shift. The donor-acceptor properties in complexes of type 8 have also been investigated by UV/vis spectroscopy and density functional theory calculations.

Full text of DOI:10.1021/acs.inorgchem.6b00917

Article
pubs.acs.org/IC
Copper(I) Complexes of Pyridine-Bridged Phosphaalkene-Oxazoline
Pincer Ligands
Spencer C. Serin, Fraser S. Pick, Gregory R. Dake,* and Derek P. Gates*
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada
S
* Supporting Information
ABSTRACT: The synthesis of enantiomerically pure pyr-
idine-bridged phosphaalkene-oxazolines ArPC(Ph)(2,6-
C5H3NOx) (1, Ar = Mes/Mes*, Ox = CNOCH(i-Pr)-
CH2/CNOCH(CH2Ph)CH2) is reported. This new ligand
forms a κ(P), κ2(NN) dimeric complex with copper(I) (7) that
dissociates into a cationic κ3(PNN) monomeric complex upon
addition of a neutral ligand {[1a·CuL]OTf (8a−e): L = PPh3
(a), P(OPh)3 (b), 2,6-lutidine (c), 4-DMAP (d), 1-
methylimidazole (e)}. The P−Cu bond lengths in 8 are
influenced by the π-accepting/σ-donating properties of L, and
this can be observed by changes in the δ31PPC NMR shift.
The donor−acceptor properties in complexes of type 8 have
also been investigated by UV/vis spectroscopy and density
functional theory calculations.
INTRODUCTION
■
The tridentate pincer motif, a classic framework, is often used
to investigate both profound and subtle steric and electronic
effects within ligand design.1 2,6-Disubstituted pyridines are
well recognized as obvious chemical systems for the creation of
tridentate pincer ligand sets. Often these pyridines are
difunctionalized with common donor sets based on N, O, S,
and P atoms. Although symmetric ligands can be easier to
synthesize, nonsymmetric 2,6-substituted pyridines offer the
possibility of fortuitous binding properties by forced orientation
of different donors in a trans-geometry within a metal complex
(e.g., A and B in Figure 1).2
There is considerable interest in the coordination chemistry
of compounds containing low coordinate phosphorus, such as
phosphaalkenes or phosphinines, for potential applications in
catalysis.3,4 The PC double bond features a low-energy π*-
orbital that leads to excellent π-accepting properties.5 The most
Figure 1. Selected examples of pincer and mixed PN phosphaalkene
studied6 phosphaalkene-containing pincer ligand motif is the
ligands.
symmetric ligand D [i.e., 2,6-bis(phosphaethylene)pyridine or
BPEP, R = H7 and R = Ph8]. This class of ligand has attracted
attention due to its highly delocalized π-electron backbone that
is not found in analogous bisphosphine derivatives. BPEP-Ph
offers distinctive metal stabilization properties that permit the
We have previously developed bidentate ligands such as the
achiral phosphaalkene-pyridine E12 which is an effective ligand
for the Pd(II)-catalyzed Overman−Claisen rearrangement of
allyl trichloroacetimidates.13 In addition, we have developed a
isolation of a four-coordinate Fe(I) species [e.g., (BPEP-
Ph)FeBr] in an unusual 15-electron configuration.8,9 Addition-
ally, BPEP-Cu(I) complexes {e.g., Cu(X)(BPEP-Ph), where X
= [PF6]− and [SbF6]−} are extremely electron-deficient,
exhibiting strong affinity toward both [PF6]− and [SbF6]−, as
a consequence of the strong π-accepting ability of the PC
bonds.10,11
modular route to the first enantiomerically pure phosphaal-
kene-oxazoline ligands (F, PhAk-Ox)14,15 and have demon-
strated their utility in the Pd(II)-catalyzed asymmetric allylic
alkylation reaction.16 Related work involves the use of the
Received: April 13, 2016
© XXXX American Chemical Society
A
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mixed phosphaalkene-imine ligand G for the Pd(II)-catalyzed
oligomerization of ethylene.17 Inspired by the ubiquity of
bisoxazoline pyridine pincer ligands (C, PyBox) in asymmetric
catalysis,18 we envisioned the development of a hybrid
phosphaalkene-pyridine-oxazoline PNN ligand (1).
In this report, we present the modular synthesis of
enantiomerically pure phosphaalkenes 1a−c (PhAk-Pyr-Ox)
as a new ligand class that effectively coordinates to Cu(I). This
hybrid PNN ligand increases the sophistication possible for
low-coordinate phosphorus compounds as well as provides a
new nonsymmetric PNN-pincer motif for application in
coordination chemistry.
carboxylic acid (2) as an off-white solid in a 68% overall yield.
Importantly, only solvent extraction was required to purify large
batches (ca. 20 g) of acid 2. Analysis of the 13C{1H} NMR
spectrum of 2 revealed the expected characteristic signals for
the CO carbon of the benzoyl and carboxylic acid (δ13C =
192.1 and 163.7) respectively.
A multistep procedure to oxazolines 3a−b from 2 was
adopted from a previous report.20 Amides 5a-b could be
prepared in a straightforward manner by the reaction of the
acid chloride derived from 2 and either L-valinol (a) or L-
phenylalaninol (b), (Scheme 2). The alcohol functions in 5a-b
could be converted to chlorides 6a-b using SOCl2 in hot
CHCl3. A traditional cyclization21 of related chlorides to form
oxazolines that used alkaline aqueous conditions (NaOH in
refluxing MeOH/H2O) was not possible in this instance due to
the hydrolytic instability of the oxazoline product. Fortunately,
cyclization was successful under anhydrous conditions using
NaH in refluxing THF, as no aqueous workup step was
required to afford ketones 3a-b. Importantly, these steps can be
performed without thorough purification of the amides 5a-b
and chlorides 6a-b.
RESULTS AND DISCUSSION
■
Phosphaalkene Synthesis and Characterization. The
general synthetic strategy toward phosphaalkene 1 is outlined
in Scheme 1. By analogy to PhAk-Ox and PyBox, the strategy
Scheme 1. Synthetic Approach to Pyridine-Bridged
Phosphaalkene-Oxazolines
Recrystallization of the oxazolines 3a-b in Et2O afforded
analytically pure ketones in 45% and 66% yield, respectively.
Compounds 3a-b were fully characterized using 1H and
13C{1H} NMR spectroscopy and mass spectrometry. Partic-
ularly diagnostic are the 13C{1H} NMR signals for the CN of
the oxazoline and CO of the ketone in 3 {δ13C [CN, C
O] = (3a): 161.9, 192.1; (3b): 162.6, 192.2}. Each ketone
displayed optical activity {[α]2D2 = 3a: −118° cm3 g−1 dm−1 (c =
2.2 × 10−1, CHCl3); 3b: −47.6° cm3 g−1 dm−1 (c = 2.8 × 10−1,
CHCl3)}. Crystals of ketone 3a were obtained by slow
evaporation of a saturated Et2O solution. The resulting solid
state molecular structure [Figure 2a], confirmed the config-
uration of the (S)-stereocenter of the amino alcohol in the
oxazoline.
takes advantage of well-established literature condensation
reactions. Specifically, the condensation of α-silylphosphides
with ketones, the phospha-Peterson reaction, generates
phosphaalkenes in high yield.12 Similarly, the (multistep)
condensation of chiral pool derived α-amino alcohols with
carboxylic acids produces oxazolines. Consequently, ketones of
type 2 are critical synthetic building blocks. Ketones of type 2
could be conveniently prepared from a mono esterified
derivative of 2,6-pyridine dicarboxylic acid.
Carboxylic acid 4 can be prepared from commercially
available 2,6-pyridinedicarboxylic acid.19 Reaction of 4 with
thionyl chloride generates an acid chloride that undergoes
Friedel−Crafts acylation with benzene in the presence of AlCl3
(Scheme 2). The acidic workup of this acylation also
hydrolyzed the methyl ester to afford 6-benzoyl-2-pyridine-
The phospha-Peterson reaction12 was performed as the PC
bond forming step (Scheme 2). The representative procedure is
as follows: a solution of ketone 3a in THF was added dropwise
to a cooled (−78 °C) solution of MesP(Li)SiMe3 in THF. The
reaction was warmed to room temperature whereupon an
aliquot was removed for analysis by 31P NMR spectroscopy.
The signal assigned to MesP(Li)SiMe3 (δ = −187) was
replaced by two new signals of equal intensity at 266.6 and
248.3 ppm, in a range consistent with those expected for
phosphaalkenes12 and indicating the product was formed as a
Scheme 2. Synthesis of Pyridine-Bridged Phosphaalkene-Oxazolines
B
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(Z)], 1H/13C NMR spectroscopy, spectroscopy and mass
spectrometry [HRMS (1b+): m/z 555.3491 (found); 555.3504
(calcd), (1c+): m/z 476.2018; 476.2018 (calcd)].
Coordination of 1a to Copper. The coordination
chemistry of PNN-proligand 1a is of considerable interest. As
a starting point, we investigated binding of 1a to copper(I)
since related NNN-pincer complexes have been used in a
variety of asymmetric transformations, such as hydrosilyla-
tions,23,24 aziridinations,25 alkyne additions to imines,26 and
conjugate additions,27,28 among others. Moreover, copper(I)
complexes of phosphaalkene-pyridine ligands have been
reported previously.7,10,11,29,30
A mixture of 1a and Cu(MeCN)4OTf in CH2Cl2 was stirred
for 2 h over which time a color change from red to deep brown
was observed (Scheme 3). Analysis of an aliquot using 31P{1H}
Scheme 3. Synthesis of Copper(I)-phosphaalkene
Complexes
Figure 2. Molecular structures of (a) 3a and (b) Z-1a. Thermal
ellipsoids are drawn at the 50% probability level. Hydrogen atoms
omitted for clarity. Selected bond lengths (Å) and angles (deg): 3a:
O(1)−C(7) 1.217(2), N(2)−C(13) 1.268(2), O(2)−C(13) 1.353(2),
C(1)−C(7)−C(8) 120.9(2), C(12)−C(13)−O(2) 117.0(1), C(12)−
C(13)−N(2) 123.5(2), O(2)−C(13)−N(2) 119.4(2). Z-1a: P(1)−
C(1) 1.693(2), P(1)−C(19) 1.834(2), C(1)−C(2) 1.488(3), C(1)−
C(8) 1.491(3), C(1)−P(1)−C(19) 104.2(1), C(2)−C(1)−P(1)
119.3(2), C(8)−C(1)−P(1) 125.4(2), and C(2)−C(1)−C(8)
115.2(2).
NMR spectroscopy revealed quantitative conversion of the
signals for both isomers of phosphaalkene 1a to a single new
resonance at 186 ppm, a value shifted considerably upfield
relative to pincer precursor E-1a (Δδ = −62). The appearance
of a single resonance in the 31P{1H} NMR spectrum of the
reaction mixture suggests isomerization of the ligand prior to
coordination. An upfield shift of >60 ppm for 1a and Cu(I) is
larger than any previously reported copper(I) phosphaalkene-
pyridine complexes [Δδ = {E-Mes*PC(2-Py)(H)}Cu-
mixture of E and Z stereoisomers. The desired phosphaalkene
1a (yield = 54%) was isolated by trituration with hexanes to
give the product as a red solid (ca. 1.0:0.64, Z:E) and
crystallographically characterized as the Z-isomer after
recrystallization from benzene [Figure 2b]. Overall the metrical
parameters are consistent with those observed previously for
Mes-substituted phosphaalkenes such as MesPCPh2 and
related systems.12,22
7
(MeCN)2 : −15 ppm; (BPEP-Ph)Cu(SbF6): −35 ppm;10
(BPEP-H)Cu(MeCN)+: −16 ppm30]. The product was
precipitated by addition of Et2O, and filtered to give complex
7 as a brown solid in 79% yield.
+
Phosphaalkene 1a was fully characterized using 1H, 13C{1H},
and 31P{1H} NMR spectroscopy, and mass spectrometry
[HRMS (1a+): m/z 428.2018 (found); 428.2016 (calcd)].
Signals could be unequivocally assigned to either the Z- or E-
Single crystals of complex 7 were obtained by slow diffusion
of Et2O into a CH2Cl2 solution. X-ray crystallographic analysis
produced the structure of an unexpected dimeric Cu(I)
complex [Figure 4a]. Phosphaalkene 1a did not coordinate to
copper as a neutral κ3(PNN) ligand as expected, but rather
κ(P), κ2(NN). Each Cu(I) is bound to an oxazoline-pyridine
moiety in addition to the phosphaalkene of a neighboring 1a
molecule. This coordination mode has not been observed for
related BPEP-Cu(I) complexes. The closest example is the
dimeric copper complex H (Figure 3) which was isolated from
the reaction of (S,S)-i-Pr-PyBox with Cu(MeCN)4PF6. Unlike
complex 7, the Cu(I) centers in H have different coordination
numbers. In each case, the pincer serves as a bridging ligand.
It should be noted that the steric environment around the
Mes-substituted phosphorus atom in 1a is smaller than that of
the Mes*-substituted phosphorus atoms within BPEP. There-
fore, ligand 1a is more likely to form a bridging dimeric
complex such as 7, while all the previously reported BPEP
1
1
isomer by using H−1H NOESY, H−1H COSY, and 31P-
{1H}-1H HSQC NMR spectroscopy (Figures S1 and S2,
Supporting Information). Consistent with previous reports of
pyridyl-substituted phosphaalkenes, the more upfield 31P{1H}
NMR signal is indicative of a Z-isomer [e.g., δ31P = MesP
CPh(2-py): 260.1 (E), 242.1 (Z);12 Mes*PCH(2-py): 285.4
(E), 259.6 (Z)7].
To illustrate the modularity of our synthetic route to chiral
phosphaalkenes, two additional phosphaalkenes 1b and 1c were
prepared in 27% and 42% yields, respectively. These
compounds were prepared following the same procedure as
described for 1a. The steric bulk of the P-substituent is
increased in 1b by employing the Mes* moiety. Additionally,
by using ketone 3b instead of 3a, a benzyl moiety (1c) was
substituted in place of the isopropyl group (1a). Each new
phosphaalkene was characterized by 31P{1H} NMR spectros-
copy [δ31P = 1b: 266.0 (E), 257.6 (Z); 1c: 264.4 (E), 245.5
C
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an optical rotation {[α]2D2 = 7: −24.7° cm3 g−1 dm−1 (c = 1.8 ×
10−1, CH2Cl2); 8a: 89.2° cm3 g−1 dm−1 (c = 0.72 × 10−1,
CH2Cl2)}. Complex 7 displayed remarkable air and moisture
stability as a solid and could be stored for weeks on the
benchtop with no signs of degradation (by 31P{1H} NMR
analysis), while 8a decomposed in ambient conditions. The
NOESY NMR spectrum of complex 7 in CDCl3 (Figure S3,
Supporting Information) suggests the dimer remains stable in
solution and does not dissociate into two monomeric copper
complexes. In particular, a NOE correlation was observed
between one ortho-CH3 group of the P-Mes substituent and
the methyl group of the oxazoline isopropyl moiety, confirming
a proximity that could only be present in the dimeric form. The
hydrogen atoms of these moieties are ca. 3.5 Å apart in the X-
ray structure of 7.
Both 7 and 8a display distorted tetrahedral geometry around
the Cu(I) center, where the dihedral angle of (P−Cu−L)−(N−
Cu−N) (L = ancillary ligand) differs significantly from 90° [ca.
78.6° (7) and 48.6° (8a)]. The large distortion of dihedral
angle observed for complex 8a rationalizes the requirement of a
bulky ancillary ligand, as 7 is able to achieve a more ideal
tetrahedral geometry when 1a binds κ(P), κ2(NN). Particularly
diagnostic are the 31P{1H} NMR shifts for the phosphaalkene
resonances in complexes 7 (186 ppm) and 8a (232 ppm). 31P
NMR chemical shifts for phosphaalkene metal complexes have
been shown to be sensitive to P−M distance,31 with shorter P−
M bonds having more upfield resonances. Complex 7 possesses
a largely upfield 31P{1H} signal and, accordingly, the P−Cu(I)
bond lengths in 7 (ca. 2.17 Å) are shorter than 8a (ca. 2.31 Å)
and other BPEP-Cu(I) complexes [e.g., (BPEP-Ph)Cu(SbF6),
P−Cu: 2.26 and δ31P = 213;10 (BPEP-H)Cu(MeCN)+, P−Cu:
2.31 Å, and δ31P = 26930].
Figure 3. Known dimeric copper pyridine-oxazoline complex.
complexes are monomeric. It was envisioned that substituting
−
the weakly coordinating OTf and MeCN ligands in 7 with a
larger neutral ligand (L, Scheme 3) would facilitate the isolation
of a monomeric [(PNN)Cu(L)]+ complex. Fittingly, upon
addition of a bulky ligand (2 equiv) to complex 7 (1 equiv) the
reaction color drastically changed from deep brown to dark red
[L = PPh3 (8a) and P(OPh)3 (8b)], dark green [L = 2,6-
lutidine (8c)], or dark blue [L = 4-DMAP (8d) and 1-
methylimidazole (8e)]. In each case, analysis of the 31P{1H}
NMR spectra of the reaction mixture showed complete
conversion of complex 7 to a new product with a distinct
chemical shift (8a: 232 ppm; 8b: 227 ppm; 8c: 215 ppm; 8d:
208 ppm; 8e: 207 ppm). Additionally, for 8a and 8b, the free
phosphine and phosphite signals (δ31P = −6 and 129,
respectively) were replaced by new resonances (δ31P = 9 and
114, respectively), which were tentatively assigned to
coordinated phosphine or phosphite.
Complex 8a was isolated and crystals suitable for X-ray
diffraction were obtained by the diffusion of hexanes into a
THF solution. Analysis of the solid state molecular structure
revealed the monomeric [(PNN)Cu(PPh3)]+ complex [Figure
4b].
Characterization of Copper Complexes. In addition to
UV/vis spectroscopic analysis of CH2Cl2 solutions of
complexes 8a−8e revealed two distinct energy transitions
[band I: 450−700 nm and band II: 350−425 nm, Figure 5].
The absorptions in the visible region of the UV spectrum are
consistent with the observed colors of 8: 8a (λmax = 540 nm,
obtaining the solid state molecular structures for complexes 7
1
and 8a, both complexes were characterized by H, 13C{1H},
19F{1H}, and 31P{1H} NMR analysis, elemental analysis (for 7)
and mass spectroscopy (for 8a). Complexes 7 and 8a showed
Figure 4. Molecular structures of (a) 7 and (b) 8a·(C4H8O)3. Thermal ellipsoids are drawn at the 50% probability level. (a) Structure truncated to
highlight the ligand−metal binding geometry and hydrogens omitted for clarity, (b) hydrogens, THF molecules, and [OTf]− counteranion omitted
for clarity. Selected bond lengths (Å) and angles (deg): 7: P(1)−Cu(2) 2.160(1), P(2)−Cu(1) 2.180(1), N(1)−Cu(1) 2.186(3), N(2)−Cu(1)
2.072(3), N(3)−Cu(2) 2.157(3), N(4)−Cu(2) 2.073(4), P(1)−C(1) 1.694(4), P(2)−C(28) 1.690(3), N(5)−Cu(1) 2.016(3), O(3)−Cu(2)
2.156(8), C(1)−P(1)−Cu(2) 119.8(2), C(28)−P(2)−Cu(1) 121.9(1), N(1)−Cu(1)−P(2) 130.84(8), N(1)−Cu(1)−N(2) 79.6(1), P(2)−
Cu(1)−N(2) 115.6(1), P(1)−Cu(2)−N(3) 136.35(8), N(3)−Cu(2)−N(4) 80.2(1), P(1)−Cu(2)−N(4) 118.6(1). 8a·(C4H8O)3: P(1)−C(1)
1.710(2), P(1)−Cu(1) 2.3119(6), P(2)−Cu(1) 2.2263(6), N(1)−Cu(1) 2.043(2), N(2)−Cu(1) 2.072(2), C(1)−C(8) 1.464(3), N(1)−C(8)
1.350(3), N(2)−C(13) 1.281(3), C(12)−C(13) 1.469(3), P(1)−Cu(1)−P(2) 106.10(2), P(1)−Cu(1)−N(1) 78.23(5), P(1)−Cu(1)−N(2)
135.94(5), N(1)−Cu(1)−N(2) 80.34(7), P(2)−Cu(1)−N(2) 115.37(5), N(1)−Cu(1)−P(2) 138.62(6), Cu(1)−P(1)−C(1) 102.70(8), P(1)−
C(1)−C(2) 127.2(2), P(1)−C(1)−C(8) 112.4(2), C(12)−C(13)−N(2) 121.9(2), Cu(1)−N(2)−C(13) 110.5(5).
D
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Figure 5. Representative UV/vis spectra of 8a−8e in CH2Cl2 solution
(all concentrations were ca. 5 × 10−5 mol L−1).
red), 8b (λmax = 513 nm, red), 8c (λmax = 587 nm, green), 8d
(λmax = 609 nm, blue), and 8e (λmax = 604 nm, blue). It is
intriguing to note that complexes with the highest energy band
I transitions (8a, 8b) also have significantly upfield 31P{1H}
NMR shifts (Table 1). Conversely, more downfield 31P{1H}
Table 1. 31P NMR Data (ppm) and λmax Values (nm) for
Band I and Band II of 8a−8e
band I: λmax
(log ε)
band II: λmax
(log ε)
Figure 6. Important molecular orbitals for the low energy transitions
of 8a and 8d.
#
L
δ31PP=C
8a
PPh3
232
227
215
208
207
540 (4.24)
513 (4.04)
487 (4.10)
609 (4.06)
604 (4.14)
373 (4.50)
341 (4.46)
386 (4.35)
400 (4.19)
392 (4.31)
8b P(OPh)3
8c 2,6-lutidine
8d 4-DMAP
8e 1-methylimidazole
ability even vs PPh3, and its composition does not change
significantly with different ancillary ligands. In each complex,
the HOMO orbital shows interactions between the pincer
ligand and the Cu(I) center. In contrast, the HOMO(−1) are
quite different for each complex. In 8a, the HOMO(−1) is
primarily phosphaalkene-based, whereas it consists primarily of
DMAP-based electron density in 8d.
Two absorption bands (I and II) were identified. For 8a,
band I includes major and minor contributions of the HOMO/
LUMO (66%), HOMO(−1)/LUMO (15%), HOMO(−3)/
LUMO (12%), and HOMO(−4)/LUMO (11%). In contrast,
band I of 8d is composed mainly of the HOMO/LUMO
transition (67%). Consistent with the blue shift in band 1 of the
UV/vis spectrum, the calculated HOMO−LUMO gap for 8a
(ΔE = 0.297 eV) is larger than for 8d (ΔE = 0.285 eV).
Moreover, the additional mixing of lower energy HOMOs into
the band I transition of 8a may also contribute to the blue shift
compared to 8d which does not show such mixing. For 8a,
band II includes a significant contribution of the HOMO(−1)/
LUMO (52%), while band II for 8d is largely based on the
HOMO(−2)/LUMO (64%) transition.
NMR shifts are observed for complexes with low energy band I
absorbances (8d, 8e). We hypothesize that an upfield chemical
shift is a consequence of a contraction of the P−Cu(I) bond
in complex 8. Such a bond contraction would be observed for
strongly donating ligands (i.e., pyridine and imidazole-based
donors) which push electron density toward the Cu(I) center
and onto π-accepting phosphaalkene ligand strengthening the
P−Cu(I) interaction. In contrast, where π-accepting ligands
[i.e., PPh3 and P(OPh)3] compete with the π-accepting
phosphaalkene ligand, weakening the P−Cu(I) interaction.
This is consistent with that discussed earlier where complex 7,
possessing a shorter P−Cu bond, was shifted upfield with
respect to 8a. Clearly, these trends are more complex, and
additional effects (e.g., sterics) cannot be disregarded. More-
over, more subtle trends in UV/vis and δ31PPC have been
reported for bis(phosphaalkene) platinum(0) complexes.32
Computational Section. To gain further insight into the
orbitals involved in the electronic transitions in complex 8 and
the trends in λmax (band 1), we modeled 8a bearing the
accepting PPh3 and 8d bearing the strongly donating 4-DMAP.
Each structure was optimized (Figures S4 and S5, Supporting
Information). In the case of 8a, the optimized structure
matched well with that obtained from X-ray crystallography. In
the previous section, we hypothesized that the more downfield
31P{1H} NMR shift for 8d is indicative of a shortened P−Cu
bond. Accordingly, the computed P−Cu(I) bonds (8a: 2.414 Å,
8d: 2.387 Å) contract when PPh3 is replaced with 4-DMAP.
Time-dependent DFT (TD-DFT) calculations were per-
formed for 8a and 8d, and the frontier molecular orbitals are
shown in Figure 6. It is important to note the LUMO is largely
composed of the π*-PC bond, confirming its π-accepting
SUMMARY
■
The synthesis of new tridentate PhAk-Pyr-Ox ligands increases
the scope ligand motifs by combining a phosphaalkene and a
chiral oxazoline. The spectroscopic properties of Cu(I)-
complexes of this new class of pincer ligand highlights the
excellent π-accepting properties of these phosphaalkene ligands.
In particular, the P−Cu bond length can be altered by
manipulating the electron density around the metal center,
which can be observed experimentally by changes in the UV/vis
spectra, with different ancillary ligands. A shorter P−Cu
bond length leads to the an upfield δ31PPC NMR chemical
shift. In addition, red-shifted bands in the UV/vis spectrum
suggest a P−Cu bond contraction. We further investigate
E
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CH2Cl2. A few drops of DMF were added, and a septum affixed to the
flask with an outlet to an external oil bubbler. (COCl)2 (3.18 mL, 4.71
g, 37.1 mmol) was added dropwise over 30 min, and the reaction was
stirred until the bubbling subsided. Volatiles were removed by rotary
evaporation to give the crude acid chloride. A 250 mL Schlenk flask
was dried and charged with L-valinol (3.43 mL, 3.18 g, 30.9 mmol),
Et3N (21.5 mL, 15.6 g, 150 mmol), and dry CH2Cl2 (100 mL). The
flask was cooled to 0 °C and a solution of the acid chloride in CH2Cl2
(50 mL) was added in one batch. The reaction was warmed to rt and
stirred 1 h. The reaction was quenched with sat. NaHCO3 (150 mL),
and the organic layer was separated. The organic layer was washed
with 1 M HCl (150 mL), separated, and dried over Na2SO4. Volatiles
were removed by rotary evaporation to give amide 5a in suitable purity
for further reaction.
this property computationally and propose that, in complexes
bearing more electron-rich ancillary ligands (i.e., 4-DMAP), the
low-lying π*PC orbital (the LUMO of these complexes) is
able to electronically strengthen the P−Cu(I) interaction.
Future studies will attempt to take advantage of the donor−
acceptor properties of these enantiomerically pure pincer
ligands in asymmetric catalysis.
EXPERIMENTAL SECTION
■
Unless stated otherwise, all manipulations were performed using
standard Schlenk or glovebox techniques under nitrogen atmosphere.
CH2Cl2 and hexanes were deoxygenated with nitrogen and dried by
passing through a column containing activated alumina. THF was
dried over sodium/benzophenone ketyl and distilled prior to use.
CD2Cl2 and CDCl3 were deoxygenated with nitrogen prior to use.
SOCl2, benzene, AlCl3, (COCl)2, Et3N, NaH, 1-methylimidazole, and
P(OPh)3 were purchased from Sigma-Aldrich and used as received.
PPh3 and 4-DMAP were recrystallized from Et2O prior to use. 2,6-
Lutidine was dried over sodium and distilled prior to use. Compounds
6-(methoxycarbonyl)picolinic acid (4),19 MesP(SiMe3)2,12
Mes*PH2,33 L-valinol,34 L-phenylanalinol,34 Cu(MeCN)4OTf35 were
synthesized according to literature procedures. NMR spectra were
recorded at 298 K on 300 or 400 MHz spectrometers (operating
1H NMR (400 MHz, CDCl3): δ = 8.28 (dd, JHH = 8, 1 Hz, 1H),
8.12 (dd, JHH = 8, 1 Hz, 1H), 8.05−7.98 (m, 4H), 7.57 (tt, JHH = 8, 1
Hz, 1H), 7.44 (t, JHH = 8 Hz, 2H), 3.90 (m, 1H), 3.72 (dd, JHH = 11, 4
Hz, 1H), 3.65 (dd, JHH = 11, 6 Hz, 1H), 3.40 (br s, 1H), 1.95 (m, 1H),
0.93 (d, JHH = 7 Hz, 3H), 0.86 (d, JHH = 7 Hz, 3H).
In a 500 mL round-bottom flask, the amide 5a synthesized in the
previous step was dissolved in 200 mL of CHCl3. SOCl2 (13.4 mL,
22.1 g, 185 mmol) was added and the flask was equipped with a reflux
condenser. The reaction was heated to reflux for 3 h and cooled to rt,
and the volatiles removed by rotary evaporation. The residual oil was
dissolved in EtOAc (50 mL) filtered through a silica plug (ca. 8″)
using EtOAc as the eluent. The EtOAc was removed by rotary
evaporation to give chloride 6a in suitable purity for further reaction.
1H NMR (400 MHz, CDCl3): δ = 8.35 (dd, JHH = 8, 1 Hz, 1H),
8.20 (dd, JHH = 8, 1 Hz, 1H), 8.08−7.98 (m, 4H), 7.57 (tt, JHH = 8, 2
Hz, 1H), 7.45 (t, JHH = 8 Hz, 2H), 4.10 (m, 1H), 3.72 (dd, JHH = 11, 4
Hz, 1H), 3.67 (dd, JHH = 11, 4 Hz, 1H), 1.97 (m, 1H), 0.96 (d, JHH = 7
Hz, 3H), 0.89 (d, JHH = 7 Hz, 3H).
A 250 mL Schlenk flask was dried and charged with chloride 6a
synthesized in the previous step and NaH (60% dispersion in oil, 1.80
g, 46.4 mmol) (*Note: for reactions involving greater than 100 mmol,
dry 95% NaH should be used as an excess of oil can impede
purification). The solids were suspended in dry THF (125 mL) and
heated to reflux for 3 h. The reaction was cooled to rt, and the sodium
salts were removed by filtration (*Note: do not use Celite or silica as a
filter medium, as accidental hydrolysis can take place). The volatiles
were removed by rotary evaporation, and the resulting oil was
triturated with pentane to give a brown/red solid. The solid was
collected by filtration and recrystallized in Et2O to give the title
compound 3a as a light yellow solid (4.12 g, 45% over three steps).
Single crystals suitable for X-ray diffraction analysis were obtained after
a concentrated solution of 3a in Et2O was left in a−30 °C freezer.
1H NMR (400 MHz, CDCl3): δ = 8.20 (d, JHH = 8 Hz, 1H), 8.13
(d, JHH = 7 Hz, 2H), 7.99 (d, JHH = 8 Hz, 1H), 7.88 (t, JHH = 8, 1H),
7.49 (t, JHH = 7, 1H), 7.39 (t, JHH = 8, 2H), 4.43 (m, 1H), 4.11 (m,
2H), 1.80 (m, 1H), 0.98 (d, JHH = 7 Hz, 3H), 0.88 (d, JHH = 7 Hz,
3H); 13C{1H} NMR (101 MHz, CDCl3): δ = 192.1, 161.9, 154.7,
145.7, 137.3, 135.4, 132.9, 131.1, 127.9, 126.0, 125.8, 72.6, 70.7, 32.6,
18.7, 18.1; HRMS (ESI): m/z 317.1260 (calcd for C18H18N2O2Na
317.1266); Anal. Calcd for C18H18N2O2: C, 73.45; H, 6.16; N, 9.52;
Found: C, 73.30; H, 6.21; N, 9.38; [α]2D2 = −118° cm3 g−1 dm−1 (c =
2.2 × 10−1, CHCl3).
1
frequency for H). 85% H3PO4 was used as an external standard (δ
0.0) for 31P{1H} NMR spectra. CFCl3 in CDCl3 1was used as an
external solvent (δ 0.0) for 19F{1H} NMR spectra. H NMR spectra
were referenced to residual protonated solvent, and 13C{1H} NMR
spectra were referenced to the deuterated solvent. Mass spectra were
recorded on a Kratos MS 50 instrument in EI mode (70 eV).
Elemental analyses were performed in the UBC Chemistry Micro-
analysis Facility. The optical rotations were measured at a
concentration in g per 100 mL, and their values (average of 10
measurements) were obtained on a Jasco P-1010 polarimeter.
Synthesis of 6-Benzoylpicolinic Acid (2). In air, a suspension of
6-(methoxycarbonyl)picolinic acid (4) (30.4 g, 168 mmol) in SOCl2
(79.2 mL, 130 g, 1.09 mol) was heated to reflux for 2 h. The residual
SOCl2 was removed by rotary evaporation, the resulting solid was
dissolved in benzene (200 mL), and the benzene was removed by
rotary evaporation (to remove additional SOCl2). This was repeated
two additional times to give the crude acid chloride. A 1-L round-
bottom flask was charged with freshly ground AlCl3 (44.8 g, 336
mmol) followed by benzene (250 mL). The suspension was heated to
50 °C, and a solution of the crude acid chloride in benzene (250 mL)
was added in one batch with vigorous stirring. The flask was equipped
with a reflux condenser and a septum affixed to the condenser with an
outlet to an external oil bubbler. The reaction was heated to reflux
until the HCl bubbling subsided (ca. 3 h). The reaction was quenched
by pouring into a 2-L Erlenmeyer flask containing 100 mL conc. HCl
in 500 g of ice. Residual solids in the reaction flask were dissolved in
EtOAc and added to the aqueous solution. The organic layer was
separated, and the aqueous layer extracted with EtOAc (3 × 200 mL).
The organic layers were combined, washed with brine (300 mL), dried
over Na2SO4, and volatiles were removed by rotary evaporation. The
resulting light brown solid was dissolved in 200 mL of CH2Cl2 and
extracted with 1 M NaOH (3 × 200 mL). The combined aqueous
extracts were acidified with 1 M HCl until cloudy and extracted with
CH2Cl2 (3 × 200 mL). The combined organic layers were dried over
Na2SO4, and volatiles were removed by rotary evaporation to give the
title compound 2 as an off-white solid (26.0 g, 68%).
Synthesis of Synthesis of (S)-PhCOC5H3N(CNOCH-
(CH2Ph)−CH2) (3b). Ketone 3b was synthesized in an analogous
fashion to 3a: picolinic acid 2 (5.60 g, 24.8 mmol), COCl2 (3.77 g,
29.7 mmol), L-phenylalaninol (3.75 g, 24.8 mmol), and Et3N (12.5 g,
1H NMR (400 MHz, CDCl3): δ = 9.08 (br s, 1H), 8.42 (d, JHH = 8
Hz, 1H), 8.27 (d, JHH = 7 Hz, 1H), 8.19 (t, JHH = 8 Hz, 1H), 7.96 (d,
JHH = 7 Hz, 1H), 7.64 (t, JHH = 8 Hz, 1H), 7.51 (t, JHH = 7 Hz, 2H);
13C{1H} NMR (101 MHz, CDCl3): δ = 192.1, 163.7, 153.5, 144.9,
139.7, 135.3, 133.5, 130.6, 128.5, 128.4, 126.2; HRMS (ESI): m/z
250.0480 (calcd for NaC13H19NO3 250.0480); Anal. Calcd for
C13H19NO3: C, 68.72; H, 3.99; N, 6.16; Found: C, 68.45; H, 4.10;
N, 6.15; Mp: 132−134 °C.
1
124 mmol) to make the corresponding pyridyl amide 5b. H NMR
(400 MHz, CDCl3): δ = 8.23 (dd, JHH = 8, 1 Hz, 1H), 8.15 (d, JHH = 9
Hz, 1H), 8.02 (dd, JHH = 8, 1 Hz, 1H), 7.98−7.88 (m, 3H), 7.56 (tt,
JHH = 8, 2 Hz, 1H), 7.43 (t, JHH = 8 Hz, 2H), 7.17−7.07 (m, 5H), 4.35
(m, 1H), 3.82−3.62 (m, 2H), 3.03−2.81 (m, 3H).
SOCl2 (17.7 g, 150 mmol) in CHCl3 (125 mL) was used to
produce the crude chloride 6b.
1H NMR (400 MHz, CDCl3): δ = 8.38 (dd, JHH = 8, 1 Hz, 1H),
8.24 (dd, JHH = 8, 1 Hz, 1H), 8.19−8.06 (m, 4H), 7.67 (tt, JHH = 7, 2
Hz, 1H), 7.54 (t, JHH = 8 Hz, 2H), 7.25−7.19 (m, 5H), 4.62 (m, 1H),
Synthesis of (S)-PhCOC5H3N(CNOCH(i-Pr)−CH2) (3a). In air,
picolinic acid 2 (7.00 g, 30.9 mmol) was suspended in 100 mL of
F
DOI: 10.1021/acs.inorgchem.6b00917
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
3.70 (dd, JHH = 11, 4 Hz, 1H), 3.60 (dd, JHH = 11, 3 Hz, 1H), 3.00 (m,
2H).
(neutral Al2O3, 1:10 EtOAc:hexanes as the eluent) to give E/Z-1b as a
yellow solid (0.59 g, 27%).
1
31P{1H} NMR (121 MHz, CDCl3): δ = 266.0 (E), 257.6 (Z); H
NaH (0.893 g, 37.2 mmol) in THF (100 mL) was used to produce
the crude product, which was recrystallized in Et2O to give the title
compound 3b as an off-white solid (5.64 g, 66% over three steps).
1H NMR (400 MHz, CDCl3): δ = 8.26 (d, JHH = 8 Hz, 1H), 8.20
(d, JHH = 8 Hz, 2H), 8.07 (d, JHH = 8 Hz, 1H), 7.95 (t, JHH = 8 Hz,
1H), 7.57 (d, JHH = 7 Hz, 1H), 7.47 (t, JHH = 8 Hz, 2H), 7.32−7.21
(m, 5H), 4.65 (m, 1H), 4.44 (t, JHH = 9 Hz, 1H), 4.22 (t, JHH = 8 Hz,
1H), 3.24 (dd, JHH = 14, 5 Hz, 1H), 2.80 (dd, JHH = 14, 9 Hz, 1H);
13C{1H} NMR (101 MHz, CDCl3): δ = 192.2, 162.6, 154.9, 145.6,
137.5, 137.5, 133.0, 131.2, 129.1, 128.4, 128.0, 126.4, 126.1, 126.0,
72.4, 67.9, 41.4; HRMS (ESI): m/z 343.1447 (calcd for C22H19N2O2
343.1451); Anal. Calcd for C22H18N2O2: C, 77.17; H, 5.30; N, 8.18;
Found: C, 77.10; H, 5.35; N, 8.17; [α]2D2 = −47.6° cm3 g−1 dm−1 (c =
2.8 × 10−1, CHCl3).
and 13C NMR assigned only for major Z-isomer: 1H NMR (400 MHz,
CDCl3): δ = 7.71 (d, JHH = 8 Hz, 1H), 7.56−7.53 (m, 2H), 7.37−7.33
(m, 3H), 7.24−7.20 (m, 3H), 6.49 (d, JHH = 8 Hz, 1H), 4.49−4.42 (m,
1H), 4.18−4.06 (m, 2H), 1.90−1.86 (m, 1H), 1.57 (s, 9H), 1.55 (s,
9H), 1.32 (s, 9H), 1.09 (d, JHH = 7 Hz, 3H), 0.97 (d, JHH = 7 Hz, 3H);
13C{1H} NMR (101 MHz, CDCl3): δ = 179.0 (d, JPC = 45 Hz), 163.0,
159.2 (d, JPC = 14 Hz), 154.6 (d, JPC = 41 Hz), 149.9, 145.8, 143.9,
143.6, 136.2, 134.7, 134.3, 129.1 (d, JPC = 8 Hz), 128.1, 127.8, 127.6,
125.9 (d, JPC = 6 Hz), 121.9, 121.3, 121.2, 72.5, 70.5, 38.1, 34.8 (d, JPC
= 15 Hz), 33.5, 33.5, 33.3 (d, JPC = 6 Hz), 32.8, 31.3; MS (ESI): m/z
579, 578 [35, 100, M-Na+]; HRMS (ESI): m/z 555.3491 (calcd for
C36H48N2OP 555.3504).
Synthesis of (S)-MesPCPhC5H3N(CNOCH(CH2Ph)−CH2)
(1c). 1c was synthesized in an analogous fashion to 1a: MesP(SiMe3)2
(1.05 g, 3.53 mmol), and ketone 6b (1.10 g, 3.21 mmol) in THF (10
mL) was used to produce the crude product, which was extracted with
toluene and washed with Et2O to give the title compound E/Z-1c as a
pink solid (0.64 g, 42%).
Synthesis of (S)-MesPCPhC5H3N(CNOCH(i-Pr)−CH2) (1a).
To a solution of MesP(SiMe3)2 (1.85 g, 6.24 mmol) in THF (10 mL)
was added methyllithium (1.6 M, 3.9 mL, 6.24 mmol). The reaction
mixture was heated to 55 °C for 1−2 h. 31P NMR analysis of an
aliquot removed from the reaction mixture suggested quantitative
formation of MesP(Li)SiMe3 (δ = −187). The reaction mixture was
cooled to −78 °C and treated with a solution of ketone 3a (1.67 g,
5.67 mmol) in THF (5 mL). After being warmed to room
temperature, analysis of an aliquot removed from the reaction mixture
by 31P NMR spectroscopy revealed two singlet resonances consistent
with phosphaalkene (E/Z-1a) (δ = 248/267 ppm). The reaction
mixture was quenched with excess SiMe3Cl (ca. 3 mL), and the solvent
was evaporated in vacuo. The product was extracted into toluene (10
mL and 2 × 5 mL) and filtered. The toluene was evaporated in vacuo,
and the resultant red paste precipitated with hexanes (10 mL). The
solid was filtered and dried in vacuo to give the product as a pink solid
(1.3 g, 54%). Single crystals of Z-1a suitable for X-ray diffraction
analysis were obtained by slow evaporation of a C6H6 solution of E/Z-
1a.
1
31P{1H} NMR (121 MHz, CDCl3): δ = 264.4 (Z), 245.5 (E); H
NMR assigned only for major E-isomer: 1H NMR (400 MHz,
CDCl3): δ = 8.07 (d, JHH = 8 Hz, 1H), 7.65 (t, JHH = 8 Hz, 1H), 7.36−
7.22 (m, 5H), 7.16 (d, JHH = 8 Hz, 1H), 7.12−7.08 (m, 3H), 6.91−
6.87 (m, 2H), 6.68 (s, 2H), 4.69−4.65 (m, 1H), 4.50 (t, JHH = 9 Hz,
1H), 4.29 (t, JHH = 8 Hz, 1H), 3.31 (dd, JHH = 13, 5 Hz, 1H), 2.79 (dd,
JHH = 14, 9 Hz, 1H), 2.25 (s, 6H), 2.19 (s, 3H); MS (EI): m/z 477,
476 [37, 100, M+], 386, 385 [20, 76, M+-CH2Ph], 240, 239 [15, 74,
+
MesPCPh+], 92, 91 [6, 45, PhCH2 ]; HRMS (EI): m/z 476.2018
(calcd for C31H29N2OP 476.2018).
Synthesis of [Cu2(1a)2(OTf)(MeCN)]OTf (7). A solution of
phosphaalkene 1a (0.40 g, 0.932 mmol) and Cu(MeCN)4OTf (0.36
g, 0.932 mmol) in CH2Cl2 (10 mL) was stirred for 2 h. The solution
was concentrated to 5 mL in vacuo, and the product was precipitated
with Et2O (ca. 10 mL). The suspension was stirred 30 min and filtered,
and dried in vacuo to give 7 as a brown solid (0.49 g, 79%). Single
crystals suitable for X-ray diffraction analysis were obtained by slow
diffusion of Et2O into a CH2Cl2 solution of 7.
1
31P{1H} NMR (162 MHz, CDCl3): δ = 248.3 (Z), 266.6 (E); H
NMR (400 MHz, CDCl3, Z-isomer): δ = 7.75 (d, JHH = 8 Hz, 1H),
7.60−7.56 (m, 2H), 7.44 (t, JHH = 8 Hz, 1H), 7.35−7.26 (m, 3H), 6.94
(d, JHH = 8 Hz, 1H), 6.61 (s, 2H), 4.45−4.39 (m, 1H), 4.19−4.07 (m,
2H), 2.32 (s, 6H), 2.13 (s, 3H), 1.90−1.83 (m, 1H), 1.04 (d, JHH = 7
31P{1H} NMR (162 MHz, CDCl3): δ = 186.0; 1H NMR (400 MHz,
CDCl3): δ = 8.04 (m, 4H), 7.43 (d, JHH = 8 Hz, 2H), 7.22 (br s, 2H),
7.1 (br s, 8H), 6.85 (s, 2H), 6.06 (s, 2H), 4.48 (t, JHH = 10 Hz, 2H),
4.37 (t, JHH = 9 Hz, 2H), 2.75 (s, 6H), 2.32 (m, 2H), 2.17 (s, 6H),
2.04 (s, 3H), 1.83 (m, 2H), 1.29 (s, 6H), 1.02 (d, JHH = 6 Hz, 6H),
0.83 (d, JHH = 7 Hz, 6H); 13C{1H} NMR (101 MHz, CDCl3): δ =
117.4 (m), 166.7, 160.4, 143.5, 142.6, 141.7, 140.7, 139.9, 130.6, 130.0,
129.9, 129.5, 129.4, 129.1, 128.7, 125.0, 74.0, 70.7, 32.3, 23.5, 21.7,
21.5, 18.9, 17.6, 2.6; 19F{1H} NMR (114 MHz, CDCl3): δ = −78.2;
Anal. Calcd for C58H61Cu2F6N5O8P2S2: C, 52.64; H, 4.65; N, 5.29;
Found: C, 52.44; H, 4.74; N, 4.93; [α]2D2 = −24.7°cm3 g−1 dm−1 (c =
1.8 × 10−1, CH2Cl2).
Synthesis of [Cu(1a)(PPh3)]OTf (8a). A solution of complex 7
(100 mg, 0.075 mmol) and PPh3 (40 mg, 0.150 mmol) in CH2Cl2 (3
mL) was stirred for 30 min. The solution was concentrated and the
resultant red residue was triturated with hexanes (2 mL). The solid
was filtered and washed with additional hexanes (2 × 2 mL) and dried
in vacuo to give 8a as a red solid (90 mg, 67%). Single crystals suitable
for X-ray diffraction analysis were obtained by slow diffusion of
hexanes into a THF solution of 8a.
1
Hz, 3H), 0.94 (d, JHH = 7 Hz, 3H); H NMR (400 MHz, CDCl3, E-
isomer): δ = 8.10 (d, JHH = 8 Hz, 1H), 7.62 (t, JHH = 8 Hz, 1H), 7.14
(d, JHH = 8 Hz, 1H), 7.12−7.06 (m, 3H), 6.92−6.88 (m, 2H), 6.68 (s,
2H), 4.62−4.55 (m, 1H), 4.30−4.22 (m, 1H), 4.14−4.07 (m, 1H),
2.26 (s, 3H), 2.25 (s, 3H), 2.18 (s, 3H), 1.93−1.86 (m, 1H), 1.10 (d,
JHH = 7 Hz, 3H), 0.98 (d, JHH = 7 Hz, 3H); 13C NMR data only
provided for the major Z-isomer:13C{1H} NMR (101 MHz, CDCl3): δ
= 191.5 (d, JPC = 41 Hz), 162.7, 160.7, 160.1, 146.0, 142.3 (d, JPC = 23
Hz), 140.4 (br s), 138.0, 136.7, 135.6, 128.9 (d, JPC = 5 Hz), 128.2,
127.5, 126.5 (d, JPC = 22 Hz), 124.7 (d, JPC = 6 Hz), 123.2, 121.8, 72.5,
70.4, 32.6, 22.4 (d, JPC = 8 Hz), 20.9, 18.8, 18.0; MS (EI): m/z [%]
430, 429, 428 [3, 29, 100, M+], 386, 385 [4, 17, M+-CHMe2]; HRMS
(EI): m/z 428.2016 (calcd for C27H29N2OP 428.2018).
Synthesis of (S)-Mes*PCPhC5H3N(CNOCH(i-Pr)−CH2) (1b).
To a cooled (−78 °C) solution of MesPH2 (1.35 g, 4.85 mmol) in
THF (15 mL) was added n-butyllithium (1.6 M in hexanes, 3.6 mL,
5.81 mmol). The reaction was warmed to room temperature and
stirred 1 h. The solution was cooled (−78 °C), and TMSCl (0.63 g,
5.81 mmol) was added. The reaction was warmed to room
temperature and immediately cooled (−78 °C), and n-butyllithium
(1.6 M in hexanes, 3.3 mL, 5.34 mmol) was added. The reaction was
stirred 1 h and ketone 6a (1.14 g, 3.88 mmol) in THF (5 mL) was
added to the cooled solution, and the mixture was allowed to warm to
room temperature. The reaction was quenched with excess TMS-Cl
(ca. 1 mL), and the solvent was evaporated in vacuo. At this point the
product could be handled under ambient moisture and air. The
product was dissolved in hexanes (ca. 3 mL) and filtered through a
small plug of Celite. Volatiles were removed using rotary evaporation,
and the yellow residue was purified by column chromatography
31P{1H} NMR (162 MHz, CD2Cl2): δ = 231.7, 8.9; 1H NMR (400
MHz, CD2Cl2): δ = 8.12 (d, JHH = 8 Hz, 1H), 8.01 (t, JHH = 8 Hz,
1H). 7.79 (d, JHH = 8 Hz, 1H), 7.43−7.21 (m, 21 H), 7.10 (br s, 2H),
6.91 (s, 1H), 6.16 (s, 1H), 4.60 (m, 1H), 4.50 (m, 1H), 2.99 (m, 1H),
2.65 (s, 3H), 2.19 (s, 3H), 1.62 (m, 1H), 1.01 (s, 3H), 0.83 (d, JHH = 7
Hz, 3H), 0.72 (d, JHH = 7 Hz, 3H); 13C{1H} NMR (101 MHz,
CD2Cl2): δ = 171.8, 167.0, 156.4 (d, JPC = 9 Hz), 144.1 (d, JPC = 20
Hz), 142.2, 140.9, 139.1, 138.6, 138.0 (d, JPC = 12 Hz), 133.7 (d, JPC
=
14 Hz), 131.3, 130.8 (d, JPC = 25 Hz), 129.5, 129.4, 129.3, 129.0, 125.6
(d, JPC = 9 Hz), 123.5, 75.6, 69.9, 33.1, 23.2 (d, JPC = 22 Hz), 22.2,
21.2, 18.5, 18.3; 19F{1H} NMR (114 MHz, CDCl3): δ = −78.6;
G
DOI: 10.1021/acs.inorgchem.6b00917
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
HRMS (EI): m/z 640.0839 (calcd for C28H29O4N2PF363CuS
640.0834); [α]D22 = 89.2° cm3 g−1 dm−1 (c = 0.72 × 10−1, CH2Cl2).
Representative Addition of a Neutral Ligand to Complex 7
(8b−e). A solution of complex 7 (3 mg, 5 μmol) and L (5 μmol) in
CH2Cl2 (0.7 mL) was sonicated for 30 min. The brown color of
complex 7 immediately changed to either a deep red (L = P(OPh)3),
deep green (L = 2,6-lutidine), or deep blue (L = 4-DMAP or 1-
methylimidazole) color. Subsequent 31P{1H} NMR analysis of the
mixture revealed quantitative conversion to a new product.
ACKNOWLEDGMENTS
■
The authors thank the Natural Sciences and Engineering
Research Council (NSERC) of Canada for financial support in
the form of Discovery, Strategic, and Research Tools &
Instruments grants to G.D. and D.G.
REFERENCES
■
31P{1H} NMR (162 MHz, CH2Cl2): δ = 227.2, 114.3 (L =
P(OPh)3), 214.7 (L = 2,6-lutidine), 208.4 (L = 4-DMAP), 207.4 (L =
1-methylimidazole).
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X-ray Crystallography. All single crystals were immersed in oil
and mounted on a glass fiber. Data were collected on a Bruker X8
APEX II diffractometer with graphite-monochromated Mo Kα
radiation. All structures were solved by direct methods and subsequent
Fourier difference techniques. All non-hydrogen atoms were refined
anisotropically with hydrogen atoms being included in calculated
positions but not refined. All data sets were corrected for absorption
effects (SADABS), Lorentz, and polarization effects. All calculations
were performed using SHELXL-2014 crystallographic software
package from Bruker AXS.36 Absolute configuration was confirmed
on the basis of the refined Flack parameter.37 Compound 7 crystallizes
with positional disorder for the chelating -OSO2CF3 ligand. Their
respective populations were refined to the final occupancy of 0.564(4)
{S(1), O(3), O(4), O(5), C(57), F(1a), F(2a), F(3a)} and 0.436(4)
{S(1a), O(3a), O(4a), O(5a), C(57a), F(1), F(2), F(3)}. Compound
8a recrystallizes with two ordered molecules of THF and a disordered
molecule of THF in the asymmetric unit, but only one atom could be
located on using the difference map and was refined anisotropically.
Their respective populations were refined to the final occupancy of
0.77(1) {C(52)} and 0.23(1) {C(52a)}. Additional crystal data and
details of data collection and structure refinement are listed in Table
S5 in the Supporting Information. All crystallographic data has been
deposited with the Cambridge Structural Database: 1469495, 1469496,
1469497, 1469498.
(2) For examples and applications of nonsymmetric PNN pincer
ligands, see: (A) (a) Cheisson, T.; Auffrant, A. Dalton Trans. 2016, 45,
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(3) For selected reviews highlighting PC bonds in coordination
Computational Details. All calculations were performed with
Gaussian 0938 using the B3LYP density function.39−42 The full
compounds were first optimized using the minimal basis set 6-
31G(d,p)43,44 using an initial geometry obtained from a solid state
molecular structure when available. Final optimization and TD-B3LYP
calculations were preformed using the superior TZVP basis set.45
chemistry, see (a) Muller, C.; Broeckx, L. E. E.; de Krom, I.; Weemers,
̈
J. J. M. Eur. J. Inorg. Chem. 2013, 2013, 187−202. (b) Dugal-Tessier, J.;
Conrad, E.; Dake, G. R.; Gates, D. P. In Phosphorus(III) Ligands in
Homogenous Catalysis: Design and Synthesis; Kamer, P. C. J., van
Leeuwen, P. W. N. M., Eds.; Wiley: Chichester, 2012; Chapter 10.
(c) Muller, C.; Vogt, D. Dalton Trans. 2007, 5505−5523. (d) Le
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Floch, P. Coord. Chem. Rev. 2006, 250, 627−681. (e) Ozawa, F.;
Yoshifuji, M. Dalton Trans. 2006, 4987−4995. (f) Weber, L. Angew.
Chem., Int. Ed. 2002, 41, 563−572.
ASSOCIATED CONTENT
■
S
* Supporting Information
(4) For recent work highlighting PC bonds in coordination
chemistry, see (a) Chang, Y.; Tanigawa, I.; Taguchi, H.; Takeuchi, K.;
Ozawa, F. Eur. J. Inorg. Chem. 2016, 2016, 754−760. (b) Klein, M.;
Schnakenburg, G.; Espinosa Ferao, A.; Streubel, R. Dalton Trans. 2016,
45, 2085−2094. (c) Rigo, M.; Sklorz, J. A. W.; Hatje, N.; Noack, F.;
Weber, M.; Wiecko, J.; Muller, C. Dalton Trans. 2016, 45, 2218−2226.
(d) Magnuson, K. W.; Oshiro, S. M.; Gurr, J. R.; Yoshida, W. Y.;
Gembicky, M.; Rheingold, A. L.; Hughes, R. P.; Cain, M. F.
Organometallics 2016, 35, 855−859. (e) Masuda, J. D.; Boere, R. T.
Dalton Trans. 2016, 45, 2102−2115. (f) Ghalib, M.; Jones, P. G.;
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b00917.
Additional NMR spectra, optimized structures of 8a and
8d, and crystallographic details (PDF)
Crystallographic data for 3a (CIF)
Crystallographic data for Z-1a (CIF)
Crystallographic data for 7 (CIF)
Crystallographic data for 8a (CIF)
Cartesian coordinates for the calculated structures 8a
(XYZ) and 8d (XYZ)
́
Schulzke, C.; Sziebert, D.; Nyulaszi, L.; Heinicke, J. W. Inorg. Chem.
2015, 54, 2117−2127. (g) Ueta, Y.; Mikami, K.; Ito, S. Inorg. Chem.
2015, 54, 8778−8785. (h) Greenacre, V. K.; Trathen, N.; Crossley, I.
R. Organometallics 2015, 34, 2533−2542. (i) Takeuchi, K.; Minami, A.;
Nakajima, Y.; Ozawa, F. Organometallics 2014, 33, 5365−5370.
(j) Broeckx, L. E. E.; Lutz, M.; Vogt, D.; Muller, C. Chem. Commun.
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2011, 47, 2003−2005.
AUTHOR INFORMATION
■
́
(5) Doux, M.; Moores, A.; Mezailles, N.; Ricard, L.; Jean, Y.; Floch,
P. L. J. Organomet. Chem. 2005, 690, 2407−2415.
Corresponding Authors
(6) Additional pyridine-based low-valent phosphorus pincer ligand
*(G.R.D.) E-mail: gdake@chem.ubc.ca.
*(D.P.G.) E-mail: dgates@chem.ubc.ca.
motifs are reported in (a) Muller, C.; Pidko, E. A.; Lutz, M.; Spek, A.
̈
L.; Vogt, D. Chem. - Eur. J. 2008, 14, 8803−8807. (b) Ekici, D. G.;
Nieger, M.; Nyulaszi, L.; Niecke, E.; Gudat, D. Angew. Chem., Int. Ed.
2002, 41, 3367−3371.
Notes
The authors declare no competing financial interest.
H
DOI: 10.1021/acs.inorgchem.6b00917
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