New Alkylidyne Complexes Featuring a Flexible Trianionic ONO3- Pincer-Type Ligand: Inorganic Enamine Effect versus Sterics in Electrophilic Additions

Article abstract of DOI:10.1021/acs.organomet.5b00155

Metal-carbon multiple bonds exhibit enhanced nucleophilicity at the α carbon within ONO trianionic pincer alkylidyne complexes. Defined as the inorganic enamine effect, this phenomenon is a result of the overlap of the N atom lone pair within the ONO^3- ligand and a π bond from the metal-carbon multiple bond. Treating the proligand [O^CH2N^CH2O]H₃ (2) with (^tBuO)₃W≡ CR (where R = Et, ^tBu) results in the formation of the dianionic pincer complexes [O^CH2NH^CH2O]W≡ CR(O^tBu) (where R = Et (3-Et), ^tBu (3-^tBu_anti)). Deprotonation of 3-^tBu_anti by treatment with Ph₃P=CH₂ forms the anionic alkylidyne complex {[O^CH2N^CH2O]W≡ C^tBu(O^tBu)}{CH₃PPh₃} (4-^tBu). DFT calculations modeling 4-^tBu reveal overlap of the N atom lone pair with a W≡ C π bond. However, 4-^tBu reacts with electrophiles preferentially at the pincer N atom as opposed to the W≡ C_α group. Multinuclear NMR spectroscopy, combustion analysis, and single-crystal X-ray crystallography are employed to characterize complexes 3-Et, 3-^tBu_anti, and 4-^tBu. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.organomet.5b00155

Article
pubs.acs.org/Organometallics
New Alkylidyne Complexes Featuring a Flexible Trianionic ONO3−
Pincer-Type Ligand: Inorganic Enamine Effect versus Sterics in
Electrophilic Additions
Sudarsan VenkatRamani, Nicholas B. Huff, Muhammad Tariq Jan, Ion Ghiviriga, Khalil A. Abboud,
and Adam S. Veige*
Department of Chemistry, Center for Catalysis, University of Florida, Gainesville, Florida 32611, United States
S
* Supporting Information
ABSTRACT: Metal−carbon multiple bonds exhibit enhanced
nucleophilicity at the α carbon within ONO trianionic pincer
alkylidyne complexes. Defined as the inorganic enamine effect,
this phenomenon is a result of the overlap of the N atom lone
pair within the ONO3− ligand and a π bond from the metal−
carbon multiple bond. Treating the proligand [OCH2NCH2O]H3
t
(2) with (tBuO)3WCR (where R = Et, Bu) results in the
formation of the dianionic pincer complexes [OCH2NHCH2O]-
WCR(OtBu) (where R = Et (3-Et), tBu (3-tBuanti)).
Deprotonation of 3-tBuanti by treatment with Ph3PCH2
forms the anionic alkylidyne complex {[OCH2NCH2O]W
CtBu(OtBu)}{CH3PPh3} (4-tBu). DFT calculations modeling
4-tBu reveal overlap of the N atom lone pair with a WC π
bond. However, 4-tBu reacts with electrophiles preferentially at the pincer N atom as opposed to the WCα group. Multinuclear
NMR spectroscopy, combustion analysis, and single-crystal X-ray crystallography are employed to characterize complexes 3-Et,
3-tBuanti, and 4-tBu.
delocalizing electron density from the nitrogen atom lone pair
onto the α carbon, and being π* in character, the HOMO
orbital is destabilized.
INTRODUCTION
■
Tailored ancillary ligands are critical to the further development
of homogeneous transition-metal catalysis.1 Ligand modifica-
tions enable exquisite control over the electronic and geometric
properties of metal complexes. Among the various classes of
ligands available, pincer and pincer-type ligands2 garner
particular attention. Emerging as a class of their own, trianionic
pincer ligands,3 thus far, have been well-suited for stabilizing
early transition metals in their high oxidation states. Trianionic
[NCN],4−7 [OCO],8−20 [NNN],21−25 [CCC],26 and
[ONO]27−36 ligands are some of the common donor motifs
and in combination with the appropriate metal ion will catalyze
nitrene and carbene transfer,21−23,27,37 aerobic oxidation,14
ethylene and alkyne polymerization,5,6,15,16,36 and alkene
isomerization.5
The trianionic pincer ligand [CF3-ONO]3−, first used to
illustrate the inorganic enamine effect, is relatively rigid and
contains a biaryl amido ligand.28−30 One question to answer is
as follows: what influence do the N atom substituents (aryl vs
alkyl) and the flexibility of the ligand have on the
nucleophilicity of metal−carbon multiple bonds? Described
herein is the synthesis of the trianionic pincer ligand precursor
[OCH2NCH2O]H3 (2), its metalation with (tBuO)3WCEt and
(tBuO)3WCtBu to afford complexes 3-Et and 3-tBuanti
,
access to the trianionic version {[OCH2NCH2O]WCtBu-
(OtBu)}{CH3PPh3} (4-tBu), and in situ solution-phase experi-
ments to assess the inorganic enamine effect on 4-tBu.
Important structure−bonding relationships29,33 within tri-
anionic pincer complexes are emerging and serve to elevate the
level of electronic and geometric control over metal ions. For
example, tungsten−alkylidene and −alkylidyne complexes
featuring the [ONO]3− pincer-type ligand [CF3-ONO]H3
exhibit enhanced nucleophilicity at the metal−carbon multiple
bond.28−30 The enhanced nucleophilicity is a consequence of
the “inorganic enamine effect” (Figure 1).28−30 Analogously to
enamines, an inorganic enamine involves constraining a
nitrogen atom lone pair to be collinear with a metal−carbon
π bond. The interaction accentuates the nucleophilicity by
RESULTS AND DISCUSSION
■
In four steps, starting from commercially available 2-(tert-
butyl)phenol, or alternatively in three steps from 3-(tert-butyl)-
2-hydroxybenzaldehyde, the proligand 238,39 was synthesized
(Scheme 1). o-Formylation of 2-(tert-butyl)phenol using the
procedure40 described by Skattebol et al. affords the
salicylaldehyde 3-(tert-butyl)-2-hydroxybenzaldehyde in gram
Received: February 24, 2015
Published: May 28, 2015
© 2015 American Chemical Society
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Scheme 2. Synthesis of [OCH2NHCH2O]WCEt(OtBu) (3-
Et) and [OCH2NHCH2O]WCtBu(OtBu) (3-tBuanti
)
ONO]H3, as result of proton migration to the alkylidyne.29
Cooling concentrated diethyl ether solutions of 3-Et and
3-tBuanti to −35 °C provides single crystals amenable to X-ray
interrogation. Figures 2 and 3 depict the solid-state molecular
structures of 3-Et and 3-tBuanti
.
During the crystallization step, 3-Et binds a diethyl ether
molecule to complete the distorted-octahedral coordination
environment around the W(VI) ion (see Figure 2, right). The
W1−C31 bond length of 1.760(2) Å and the ∠W1−C31−C32
bond angle of 178.11(18)° are consistent with those of known
high-oxidation-state tungsten− and molybdenum−alkylidynes
featuring a pincer ligand (either dianionic or tria-
nionic).15,29,30,32,44 The sum of angles around the central
nitrogen donor of 339.33(13)° implies a pyramidal geometry;
indeed, the difference Fourier Map reveals electron density for
a proton on the nitrogen that was refined freely. The long W1−
N1 bond distance of 2.2743(18) Å is consistent with a
protonated “amino” rather than “amido” nitrogen, which serves
as an L-type donor.
Figure 1. Enhancement of metal−carbon multiple bond nucleophil-
icity due to overlap of the amido lone pair with the tungsten alkylidyne
π bonds within [CF3-ONO]WCEt(OtBu)− (orbital pictures
presented at isovalue 0.056187).
quantities and sufficient purity. Methylation of the phenolic
oxygen followed by reductive amination using the Williamson
method41 provides the protected secondary amine derivative 1
in reasonable yield and purity. Demethylation using BBr3 in
CH2Cl2 at 0 °C, followed by successive acid and base washes
and a pentane recrystallization of the crude mixture, provides
access to the proligand 2 in 16−25% overall yield. The low
yield is a consequence of the BBr3 deprotection step. Fresh
BBr3 must be used to achieve the modest yields. The 1H NMR
spectrum of 2 in CDCl3 is characteristic of a C2-symmetric
molecule. The methylene spacer in 2 resonates as a singlet,
integrating to four protons at 3.94 ppm. The other resonances
pertaining to the tert-butyl group and the aromatic protons are
unexceptional.
1
A H NMR spectrum (C6D6) of recrystallized 3-Et exhibits
signals consistent with a Cs-symmetric complex. The higher
symmetry is in contrast with the C1-symmetric structure
observed in the solid state. Presumably, a fluxional process flips
the C14 methylene from the up position to the down position
and vice versa for C7. The dynamic process occurs more
quickly than the NMR time scale, therefore imparting a mirror
plane that bisects the complex. As a consequence, the two −tBu
groups from the pincer ligand are symmetric and resonate as a
singlet at 1.63 ppm. The −OtBu protons resonate slightly
t
downfield of the pincer ligand Bu protons at 1.78 ppm. The
alkylidyne ethyl group CH2 protons appear as a quartet
centered at 3.85 ppm, and the CH3 protons resonate upfield at
0.69 ppm. The methylene protons on the pincer ligand are
diastereotopic and exhibit different scalar couplings with the
amino proton. The net result is the appearance of two doublets
Syntheses and Solid-State Structures of
t
[OCH2NHCH2O]WCR(OtBu) (3-R; R = Et, Bu). Combining
solutions of [OCH2NCH2O]H3 (2) and (tBuO)3WCEt42 or
(tBuO)3WCtBu43 in benzene generates the dianionic pincer-
type complexes [OCH2NHCH2O]WCEt(OtBu) (3-Et) and
[OCH2NHCH2O]WCtBu(OtBu) (3-tBuanti), respectively
(Scheme 2). Complexes 3-Et and 3-tBuanti retain the proton
on the N atom, and this is the first clear indication that ligand 2
is significantly different from the rigid [CF3-ONO]H3 ligand.29
The alkylidenes [CF3-ONO]WCHEt(OtBu) and [CF3-
ONO]WCHtBu(OtBu) form upon metalation with [CF3-
2
of doublets: one centered at 3.33 ppm (with JHH = 13.1 Hz
3
and JHH = 2.4 Hz) and another centered at 4.51 ppm (with
2JHH = 13.1 Hz and 3JHH = 12.2 Hz). The protons that resonate
at 4.51 ppm must be anti relative to the NH due to its large
coupling constant (12.2 Hz). In contrast, the coupling constant
for the syn protons is only 2.4 Hz.45,46 A resonance at 283.5
a
Scheme 1. Synthesis of [OCH2NCH2O]Me2 (1) and [OCH2NCH2O]H3 (2)
a
Legend: (i) Ti(OiPr)4, NH4Cl, NEt3, NaBH4, ethanol (200 proof); (ii) BBr3, CH2Cl2, 0 °C.
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Figure 2. (left) Solid-state structure of 3-Et with hydrogen atoms (except H1) and disordered C33 removed for clarity. (right) Truncated thermal
ellipsoid plot (40% probability) of the W(VI) core. Selected bond distances (Å): W1−C31−1.760(2), W1−O1−1.9918(13), W1−O2−1.9929(14),
W1−O3−2.4973(14), W1−O4−1.9004(14), W1−N1−2.2743(18), N1−H1−0.83(2). Selected bond angles (deg): ∠W1−C31−C32−178.11(18),
∠C31−W1−O3−172.32(7), ∠N1−W1−O4−158.59(6), ∠O1−W1−O2−150.42(6), ∠C14−N1−C7−110.03(16), ∠C14−N1−W1−112.10(12),
∠C7−N1−W1−117.20(12).
Figure 3. (left) Solid-state structure of 3-tBuanti with hydrogen atoms (except H1) and lattice solvent diethyl ether removed for clarity. (right)
Truncated thermal ellipsoid plot (40% probability) of the W(VI) core. Selected bond distances (Å): W1−C23−1.761(2), W1−O1−1.9992(14),
W1−O2−1.9956(13), W1−O3−1.8956(14), W1−N1−2.2709(18), N1−H1−0.82(2). Selected bond angles (deg): ∠W1−C23−C24−177.82(16),
∠N1−W1−O3−155.08(7), ∠O1−W1−O2−149.96(6), ∠C18−N1−C7−109.03(17), ∠C18−N1−W1−113.70(13), ∠C7−N1−W1−116.91(13).
t
1
ppm in the 13C{1H} NMR spectrum of 3-Et (C6D6) for W
stopped early and BuOH is removed in vacuo, a H NMR
spectrum of the resulting solid only displays signals for 3-tBuanti
with little or no resonances for 3-tBusyn. The cross peaks in the
gHMBC spectrum of the mixture indicate that both compounds
are Cs symmetric with an intact NH bond, a tBu alkylidyne, and
Cα confirms the presence of an alkylidyne.
Adding proligand 2 to (tBuO)3WCtBu in C6D6 initially
1
provides 3-tBuanti. Monitoring the reaction overtime by H
NMR spectroscopy reveals signals attributable to 3-tBuanti and
the syn isomer 3-tBusyn (Scheme 3). When the reaction is
t
a BuO ligand. Given the C2 symmetry of the ligand, the only
possible isomers are those resulting from the relative
orientation of the NH hydrogen and the alkylidyne/tBuO. An
Scheme 3. Interconversion of 3-tBuanti and 3-tBusyn
NOE between the protons in the Bu alkylidyne of 3-tBuanti
t
(0.95 ppm) and the methylene protons anti to the NH (5.09
ppm; Figure S43 in the Supporting Information), confirms the
anti disposition of the NH protons relative to the alkylidyne.
Heating a solution of 3-tBuanti for 9 days at 70 °C establishes
equilibrium between the isomers in an anti:syn ratio of 36:64,
respectively.
Depicted in Figure 3 is the solid-state molecular structure of
3-tBuanti. Complex 3-tBuanti does not bind a diethyl ether
molecule during crystallization. A simple explanation is that the
∠N1−W1−O3 angle in 3-tBuanti is 3.51(9)° smaller than that
in 3-Et, thus precluding the diethyl ether from binding. At 0.09,
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the Addison parameter47 τ is consistent with a square-pyramidal
geometry. Around the nitrogen atom, the sum of angles
measures 339.64(14)°, indicating pyramidalization, and again,
the proton was refined freely (N1−H1 = 0.82(2) Å).
Consistent with 1D-NOE data (see Figure S43 in the
Supporting Information), the proton on the nitrogen adopts
an anti configuration with respect to the alkylidyne fragment.
From previous work,29 using the more rigid [CF3-ONO]H3
ligand, we observed facile proton migration from the central
nitrogen to the alkylidyne carbon upon metalation, leading to
alkylidenes.29,30,32 3-Et and 3-tBuanti/3-tBusyn are interesting
because the −NH proton does not migrate to the alkylidyne. A
straightforward explanation is that the N group in
[OCH2NCH2O]H3 is an aliphatic secondary amine and is
therefore more basic and the flexibility within the ligand
framework places the NH anti to the alkylidyne, thereby
precluding activation. Other alkylidynes that retain an −NH
bond include Mo(NHArCl)(CtBu)[biphen]48 and W(NHAr)-
(CtBu)[OCMe(CF3)2]2(DME);49 these alkylidynes do not
interconvert to their corresponding alkylidene tautomers
Mo(NArCl)(CHtBu)[biphen]48 and W(NAr)(CHtBu)-
[(OCMe(CF3)2]2.49
Figure 4. Molecular structure of 4-tBu with hydrogen atoms and
lattice solvent benzene removed for clarity. Selected bond distances
(Å): W1−C23−1.758(4), W1−O1−2.011(3), W1−O2−2.018(3),
W1−O3−1.934(2), W1−N1−2.026(3), N1−C7−1.469(5), N1−
C18−1.466(5) P1−C50−1.785(5). Selected bond angles (deg):
∠W1−C23−C24−173.0(3), ∠N1−W1−O3−151.29(13), ∠O1−
W1−O2−146.47(11), ∠C18−N1−C7−113.9(3), ∠C18−N1−W1−
118.2(3), ∠C7−N1−W1−116.8(3).
Synthesis and Molecular Structure of {[P(CH3)-
(C6H5)3]}{[OCH2NCH2O]WCtBu(OtBu)} (4-tBu). Treating
50
3-tBuanti with freshly prepared Ph3PCH2 in pentane at
ambient temperature followed by overnight stirring precipitates
4-tBu as a fine orange powder (Scheme 4). The syntheses of
trianionic pincer complexes [tBuOCO]WCtBu(OtBu)15 and
[CF3-ONO]WCtBu(OtBu)30 employ a similar deprotona-
tion strategy.
W(VI) ion in a distorted-square-pyramidal geometry (τ =
0.08)47 and a triphenylmethylphosphonium countercation. A
striking feature found in the solid-state structure of 4-tBu is the
sum of angles around the central nitrogen donor. In spite of
being sp2 hybridized, the N atom substituents deviate from
planarity with angles adding up to only 348.4(3)°; this is
10.4(4)° short of the corresponding angles (358.8(3)°) found
in the neutral alkylidyne complex [CF3-ONO]WCtBu-
(OEt2).30 Nonetheless, other data support the nitrogen as
being sp2 hybridized. The difference Fourier map did not reveal
electron density attributable to a proton on the nitrogen;
attempts to place a proton riding on the nitrogen atom led to
chemically meaningless N−H bond lengths. In addition, the
W1−N1 bond length of 2.026(3) Å in 4-tBu is in agreement
with the corresponding bond distance of 2.008(2) Å found in
[CF3-ONO]WCtBu(OEt2)30 and the 2.161(3) Å W1−N1
bond distance in {[P(CH3)(C6H5)3]}{[pyr-ONO]WCtBu-
(OtBu)}.32 Conversely, in 3-Et and 3-tBuanti, which feature an
sp3-hybridized N atom, the W1−N1 bond distances are longer
at 2.2743(18) and 2.2709(18) Å, respectively.
Ground-State DFT Study of {[P(CH3)(C6H5)3]}-
{[OCH2NCH2O]WCtBu(OtBu)} (4-tBu). Using the hybrid
functional B3LYP51,52/LANL2DZ53 and M0654/LANL2DZ53
basis sets from the Gaussian 09 program suite,55 geometry
optimization and single point analysis of 4-tBu were performed
using spin-restricted density functional theory (DFT). Atomic
coordinates from the crystal structure serve as the initial input
for the geometry-optimized structure of 4-tBu. 4-tBu′ is
calibrated by comparison to the experimental bond lengths
and angles determined for 4-tBu. Table 1 gives pertinent bond
lengths and angles for 4-tBu and 4-tBu′. Molecular orbitals
generated from the program Gabedit are reported at their
stated isovalues.
Scheme 4. Synthesis of
{[P(CH3)(C6H5)3]}{[OCH2NCH2O]WCtBu(OtBu)}
(4-tBu)
The 1H NMR spectrum (C6D6) of 4-tBu exhibits resonances
consistent with the successful deprotonation event. The
t
resonances of the Bu directly bound to the WC unit, the
methylene, and the aryl protons from the phosphonium
countercation are broadened. Perhaps as a consequence of
this broadening and fast relaxation, the resonance correspond-
ing to WCα was not located either in a 13C{1H} or in a 2D
1H−13C gHMBC NMR experiment. Evidence from the
31P{1H} NMR spectrum for the deprotonation event comes
in the form of a singlet at 21.3 ppm corresponding to the
phosphonium countercation. This value in the 31P{1H} NMR
spectrum of 4-tBu is consistent with other anionic alkylidynes
generated employing a similar deprotonation strategy.29,30,32
Conclusive evidence for the identity of 4-tBu comes from an
X-ray crystallography experiment performed on single crystals
that grow via vapor diffusion of pentane into a benzene solution
of 4-tBu. Depicted in Figure 4 is the thermal ellipsoid plot of
the solid-state molecular structure of 4-tBu. Complex 4-tBu is
C1 symmetric in the solid state, featuring a core comprising the
As is evident from Figure 5 (only the B3LYP result
depicted), the calculation overestimates the bond lengths by
∼0.02 Å. In addition, the sum of the angles (358.15°) around
the nitrogen atom within 4-tBu′ is larger than the
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version involves removal of the bound −OtBu ligand. For
example, treating {[P(CH3)(C6H5)3]}{[CF3-ONO]WCtBu-
(OtBu)} with MeOTf in Et2O precipitates CH3PPh3OTf and
releases MeOtBu to produce the neutral alkylidyne [CF3-
ONO]WCtBu(OEt2).30 Treating 4-tBu with methylating
reagents MeOTf and Me3OBF4 in C6D6 (Scheme 5) results in a
color change from orange to yellow and precipitation of
Table 1. Selected Bond Lengths (Å) and Angles (deg) for the
Single-Crystal X-ray Structure of 4-tBu and DFT Geometry
Optimized Structure of 4-tBu′
4-tBu′ (B3LYP/
4-tBu′ (M06/
bond length
4-tBu
LANL2DZ)
LANL2DZ)
W1−O1
W1−O2
W1−O3
W1−N1
W1−C23
C23−C24
N1−C7
2.011(3)
2.018(3)
1.934(2)
2.026(3)
1.758(4)
1.515(6)
1.469(5)
1.466(5)
2.038
2.045
1.950
2.014
1.779
1.505
1.478
1.467
2.018
2.026
1.947
2.020
1.767
1.495
1.470
1.462
1
CH3PPh3OTf. The H NMR spectrum after addition reveals
the formation of several unidentifiable compounds. However,
within the mixture are resonances attributable to N-
methylation. In addition, deprotonation occurs with an instant
color change to deep brown upon dissolving 4-tBu in CDCl3 or
CHCl3 to give 3-tBu (Scheme 5). Unfortunately, we were
unable to conclusively identify the conjugate base product from
the deprotonation of either CDCl3 or CHCl3.
N1−C18
4-tBu′ (B3LYP/
4-tBu′ (M06/
bond angle
4-tBu
LANL2DZ)
LANL2DZ)
Preferential attack at the N atom over the −OtBu group is
understandable. The inorganic enamine effect increases the
nucleophilicity of the alkylidyne α carbon, but sterics play an
∠W1−C23−C24
∠N1−W1−O3
∠O1−W1−O2
∠C18−N1−C7
∠C18−N1−W1
∠C7−N1−W1
∠W1−O3−C28
173.0(3)
151.29(13)
146.47(11)
113.9(3)
118.2(3)
116.8(3)
139.3(3)
177.23
148.42
150.19
117.61
119.92
120.52
146.48
170.51
147.82
153.26
117.49
119.51
122.44
140.63
important role as well. For example, treating the ethyl
29
alkylidyne {[CF3-ONO]WCEt(OtBu)}−
with MeOTf
results in attack at the α carbon of the alkylidyne, whereas in
30
the larger Bu derivative {[CF3-ONO]WCtBu(OtBu)}−
t
attack occurs at the −OtBu group. For 4-tBu, though the
inorganic enamine effect may even be enhanced relative to that
for {[CF3-ONO]WCtBu(OtBu)}− (see strong overlap in
HOMO(-3) of Figure 6), the most open site is the nitrogen
atom. The lack of steric encumbrance around the N atom
renders it more reactive in comparison to WCα. Attack at
nitrogen in the presence of alkoxide and alkylidyne ligands is
normal. For example, electrophilic additions occur preferen-
tially at the aryl−amido ligand for the imido alkylidyne anion
{(NAr)MoCtBu[OCMe(CF3)2]2}−.57 In addition, trianionic
24
pincer-type complexes of the general formula [NNN]TaMe2
exhibit divergent reactivity as a function of π bonding between
the equatorial nitrogen atom and the Ta ion. [NNN]TaMe2
complexes with significant π bonding from the central N atom
do not react with protons or Lewis acids, whereas complexes
with minimal π bonding do.
Figure 5. Geometry-optimized structure of 4-tBu′ using B3LYP/
LANL2DZ level theory.
CONCLUSION
■
The new flexible pincer [OCH2NCH2O]H3 (2) is a viable ligand
for stabilizing tungsten alkylidyne complexes in both the
dianionic ([OCH2NHCH2O]WCEt(OtBu) (3-Et) and
[OCH2NHCH2O]WCtBu(OtBu) (3-tBuanti/3-tBusyn)) and tri-
anionic forms ({[OCH2NCH2O]WCtBu(OtBu)}{CH3PPh3}
(4-tBu)). The methylene spacers in [OCH2NCH2O]3− allow
flexibility in the backbone to maximize overlap between the N
atom lone pair and an alkylidyne π bond. The intent is to
accentuate the inorganic enamine effect and therefore the
nucleophilicity of the alkylidyne α carbon. At least from
computation results, the flexible ligand does increase the orbital
overlap between the N atom lone pair and the filled metal−
carbon π orbital in {[OCH2NCH2O]WCtBu(OtBu)}-
{CH3PPh3} (4-tBu) versus {[CF3-ONO]WCtBu(OtBu)}-
{CH3PPh3} (see Figure 6). However, attack at the exposed N
atom in the backbone on 4-tBu occurs for both H+ and Me+,
thus precluding the isolation of a neutral alkylidyne complex.
experimentally determined value of 348.4(3)°. In spite of these
differences, the calculation correctly reproduces the exper-
imentally observed twist in the ONO ligand backbone.
Evidence for the twist comes from the dihedral angle between
the N atom lone pair and the W1−C23 bond. A vector
perpendicular to the C7−W1−C18 plane within 4-tBu
represents the idealized position of the N atom lone pair. In
the crystal structure, the vector subtends 44.02(4)° with respect
to the W1−C23 bond, and the computed structure matches
with an angle of 44.32°. As a consequence, the N atom lone
pair strongly overlaps with the π bond in the WCtBu
fragment, giving rise to an “inorganic enamine” (Figure 6;
HOMO(-3)). The corresponding antibonding combination is
the HOMO and is analogous to the HOMO orbital observed
for a typical organic enamine.56 The overlap between the amido
lone pair and the alkylidyne π bonds raises the energy of the
HOMO and delocalizes electron density onto the alkylidyne α
carbon, thus increasing the nucleophilicity of the metal−carbon
multiple bond (similar to the increased nucleophilicity of an
enamine’s β carbon).
EXPERIMENTAL SECTION
■
General Considerations. Unless specified otherwise, all manip-
ulations were performed under an inert atmosphere using standard
Schlenk or glovebox techniques. Pentane, hexanes, toluene, diethyl
ether, tetrahydrofuran, and acetonitrile were dried using a Glass-
Ligand-Centered Reactivity. An established protocol15,29
to convert the anionic alkylidyne into the corresponding neutral
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Article
Figure 6. Truncated molecular orbital diagrams (B3LYP/LANL2DZ) exhibiting inorganic enamine bonding combinations for 4-tBu′ (left) and
{[CF3-ONO]WCEt(OtBu)}− anion (right). The HOMO(-3) of 4-tBu′ exhibits more overlap between the amido lone pair and a tungsten
alkylidyne π bond. The comparative bonding combination within [CF3-ONO]WCEt(OtBu)− is represented by HOMO(-2) (isovalue 0.056187).
vacuum-transferred and stored over 4 Å molecular sieves. CDCl3
(Cambridge Isotopes) was dried over CaH2, distilled, and stored over
4 Å molecular sieves. THF-d8 (Cambridge Isotopes) was used as
received. (tBuO)3WCtBu,43 (tBuO)3WCEt,42 3-(tert-butyl)-2-
hydroxybenzaldehyde,40 3-(tert-butyl)-2-methoxybenzaldehyde,58 and
Scheme 5. Nitrogen-Centered Reactivity: MeOTf Addition
and Protonation of the Anionic Alkylidyne 4-tBu in C6D6
50
Ph3PCH2 were prepared according to published procedures. All
other reagents were purchased from commercial vendors and used
1
without further purification. H and 13C{1H} 2D NMR spectra were
obtained on an Inova 500 MHz spectrometer, and 31P{1H} spectra
were acquired on either a Varian Mercury broad band 300 MHz or
Varian Mercury 300 MHz spectrometer. The chemical shifts are
reported in δ (ppm) and were referenced to the lock signal on the
1
1
TMS scale for H and 13C NMR spectra. For H and 13C{1H} NMR
spectra, the residual solvent peak was used as an internal reference.
Elemental analyses were performed at Complete Analysis Laboratory
Inc., Parsippany, NJ.
DFT Calculations. Geometry optimization and single point
analysis of 4-tBu′ were performed using spin-restricted density
functional theory calculations, with the hybrid functional
B3LYP51,52/LANL2DZ53 and M0654/LANL2DZ53 basis sets as
implemented in the Gaussian 09 program suite55. The atomic
coordinates from the crystal structures were used as an initial input
for the geometry-optimized structures. Molecular orbital pictures were
generated from Gabedit at their reported isovalues.
Contour drying column. Benzene-d6 and toluene-d8 (Cambridge
Isotopes) were dried over sodium benzophenone ketyl, distilled, or
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Synthesis of Bis(3-(tert-butyl)-2-methoxybenzyl)amine,
[OCH2NHCH2O]Me2 (1). In a round-bottom flask were placed 3-(tert-
butyl)-2-methoxybenzaldehyde (8.22 g, 43.0 mmol), Ti(OiPr)4 (25.6
mL, 85.0 mmol, 2.0 equiv), ammonium chloride (4.57 g, 85.0 mmol,
2.0 equiv), and trimethylamine (12.0 mL, 85.0 mmol, 2.0 equiv) in
that order, and the mixture was slurried using absolute ethanol. The
reaction mixture was stirred overnight under a positive pressure of
argon; the solution color changed from yellow-orange to deep orange.
NaBH4 (2.42 g, 64 mmol, 1.5 equiv) was then added as a solid to the
reaction mixture, and this mixture was further stirred for another 7 h.
The reaction mixture was quenched by pouring into ammonium
hydroxide (2 M, 200 mL); the copious white precipitate that formed
was filtered off and extracted with ethyl acetate (×3). The aqueous
layer was extracted with ethyl acetate, and the organic fractions were
combined, dried over MgSO4, and concentrated in vacuo to afford an
orange oil (4.5 g, crude). Hexane was added until all the oil dissolved;
2 M HClaq was added carefully, and the mixture was stirred for 15 min.
While still warm, the layers were separated. A white solid precipitated
from the organic layer upon cooling. The solid was filtered off and
extensively dried to give 1·HCl (yield 2.60 g, 30%). 1H NMR (CDCl3,
300 MHz): δ 7.50 (dd, 2H, 3JHH = 7.4 Hz, 4JHH = 1.7 Hz, Ar H), 7.36
(dd, 2H, 3JHH = 7.9 Hz, 4JHH = 1.7 Hz, Ar H), 7.07 (t, 2H, 3JHH = 7.6
Hz, Ar H), 4.18 (s, 4H, −NCH2), 3.55 (s, 6H, −OCH3), 1.36 (s, 18H,
−C(CH3)3) ppm.
3JHH = 12.3 Hz, −NCH2), 3.30 (dd, 2H, 2JHH = 12.1 Hz, 3JHH = 2.2 Hz,
−NCH2), 1.81 (s, 9H, −OC(CH3)3), 1.63 (s, 18H, Ar C(CH3)3), 0.98
(s, 9H, −WCC(CH3)3) ppm. 13C{1H} NMR (C6D6, 125 MHz): δ
288.9 (s, WCtBu), 161.9 (s, Ar C), 139.3 (s, Ar C), 127.5 (s, Ar C),
127.2 (s, Ar C), 122.5 (s, Ar C), 118.6 (s, Ar C), 80.5 (s,
−OC(CH3)3), 66.2 (s, WCC(CH3)3), 59.4 (s, −NCH2), 50.8 (s,
Ar C(CH3)3), 35.8 (s, WCC(CH3)3), 34.2 (s, −OC(CH3)3), 30.6
(s, Ar C(CH3)3) ppm. Anal. Calcd for C31H47NO3W: C, 55.94; H,
7.12; N, 2.10. Found; C, 55.86; H, 7.10; N, 2.09.
1
Data for 3-Et (yield 0.11 g, 57%) are as follows. H NMR (C6D6,
300 MHz): δ 7.40 (dd, 2H, 3JHH = 7.6 Hz, 4JHH = 1.8 Hz, Ar H), 6.82
3
3
(t, 2H, JHH = 7.6 Hz, Ar H), 6.75 (dd, 2H, JHH = 7.3 Hz, 4JHH = 1.5
2
3
Hz, Ar H), 4.51 (dd, 2H, JHH = 13.1 Hz, JHH = 12.2 Hz, −NCH2),
3.85 (q, 2H, WCC(CH2CH3)3), 3.33 (dd, 2H, 2JHH = 13.1 Hz, 3JHH
= 2.4 Hz, −NCH2), 1.78 (s, 9H, −OC(CH3)3), 1.63 (s, 18H, Ar
C(CH3)3), 0.69 (t, 3H, −WC(CH2CH3)) ppm. 13C{1H} NMR
(C6D6, 75 MHz): δ 283.5 (s, WCEt), 162.1 (s, Ar C), 139.4 (s, Ar
C), 127.4 (s, Ar C), 127.3 (s, Ar C), 122.9 (s, Ar C), 118.8 (s, Ar C),
81.4 (s, −OC(CH3)3), 59.4 (s, −NCH2), 55.7 (s, Ar C(CH3)3), 35.8
(s, −OC(CH3)3), 33.9 (s, WC(CH2CH3)), 30.7 (s, Ar C(CH3)3)
18.0 (s, WC(CH2CH3)) ppm. Anal. Calcd for C29H43NO3W: C,
54.64; H, 6.80; N, 2.20. Found; C, 54.09; H, 6.37; N, 2.12.
Synthesis of {[P(CH3)(C6H5)3]}{[OCH2NCH2O]WCtBu(OtBu)}
(4-tBu). A pentane solution (5 mL) of Ph3PCH2 (0.044 g, 0.16
mmol, 1.01 equiv) was added dropwise to a stirred pentane solution of
3-tBuanti (0.105 g, 0.157 mmol, 1.0 equiv), resulting in the
precipitation of a pale orange powder. The mixture was stirred for 4
h ,and the solid was separated by filtration and washed with fresh
pentane. The solid was dried under vacuum for 1 h to afford 4-tBu as a
1·HCl was slurried in diethyl ether and treated with saturated
aqueous NaOH (1 M), upon which the organic layer turned turbid; at
this point the layers were separated. The organic layer was collected,
1
dried over MgSO4 and concentrated in vacuo to yield neutral 1. H
3
4
NMR (CDCl3, 300 MHz): δ 7.29 (dd, 2H, JHH = 7.4 Hz, JHH = 1.8
1
Hz, Ar H), 7.24 (dd, 2H, 3JHH = 7.9 Hz, 4JHH = 1.8 Hz, Ar H), 7.02 (t,
fine pale orange powder (yield 0.13 g, 89%). H NMR (C6D6, 500
MHz): δ 7.55 (br, 4H, Ar H), 7.40 (dd, 2H, 3JHH = 7.1 Hz, 4JHH = 2.5
3
2H, JHH = 7.6 Hz, Ar H), 3.89 (s, 4H, −NCH2), 3.77 (s, 6H,
3
−OCH3), 1.40 (s, 18H, −C(CH3)3) ppm. 13C{1H} NMR (CDCl3, 75
MHz): δ 158.3 (s, Ar), 142.7 (s, Ar), 133.9 (s, Ar), 128.6 (s, Ar), 126.0
(s, Ar), 123.5 (s, Ar), 61.9 (s, OCH3), 49.0 (s, NCH2), 35.1 (s,
C(CH3)3), 31.1 (s, C(CH3)3) ppm.
Hz, Ar H), 6.96−7.08 (br, 8H, Ar H), 6.80 (dd, 2H, JHH = 7.4 Hz,
4JHH = 2.5 Hz, Ar H), 6.77 (t, 2H, 3JHH = 7.4 Hz, Ar H), 3.47 (br, 4H,
−NCH2), 1.82 (s, 9H, −OC(CH3)3), 1.70 (s, 18H, Ar C(CH3)3), 0.72
(s, 9H, WCC(CH3)3) ppm. 31P{1H} NMR (C6D6, 121 MHz): δ
21.3 ppm. 13C{1H} NMR (C6D6, 125 MHz): δ not observed (W
CtBu), 165.2 (s, Ar C), 138.2 (s, Ar C), 134.5 (s, Ar C), 133.1 (d, Ar C,
3JCP = 9.10 Hz), 129.5 (d, Ar C, 2JCP = 11.2 Hz), 128.7 (s, Ar C), 127.7
(s, Ar C), 126.7 (s, Ar C), 125.2 (s, Ar C), 119.1 (s, Ar C), 80.5 (s,
OC(CH3)3), 56.7 (NCH2), 50.8 (s, WCC(CH3)3), 35.8 (s, Ar
C(CH3)3), 33.6 (s, OC(CH3)3), 33.1 (s, WCC(CH3)3), 31.2 (s, Ar
C(CH3)3) ppm. Anal. Calcd for C50H64NO3PW: C, 63.76; H, 6.85; N,
1.49. Found; C, 63.42; H, 6.60; N, 1.63.
Synthesis of 6,6′-(Azanediylbis(methylene))bis(2-(tert-
butyl)phenol), [OCH2NCH2O]H3 (2). [OCH2NHCH2O]Me2 (1; 4.5 g,
0.012 mol) was dissolved in dichloromethane and cooled in an ice−
water bath. Freshly purchased BBr3 (10 mL, 8 equiv) was then
syringed carefully into the reaction vessel, leading to an immediate
color change from orange to yellow. The reaction mixture was stirred
under a positive pressure of argon at the same temperature and slowly
warmed to ambient temperature. The reaction mixture was stirred for
an additional 6 h and then cooled back to 0 °C using an ice−water
bath and carefully quenched with an ice-cold solution of methanol.
HClaq (1 M) was then added to the reaction mixture and stirred for 30
min. The layers were separated, and the organic fraction was treated
with aqueous NaHCO3. Ether extraction of the organic layer followed
by drying over MgSO4 and concentration in vacuo afforded a greasy
solid. Recrystallization from cold pentane provided pure 2 (yield 0.67
g, 16%). 1H NMR (CDCl3, 300 MHz): δ 7.25 (dd, 2H, 3JHH = 7.6 Hz,
4JHH = 1.5 Hz, Ar H), 6.97 (dd, 2H, 3JHH = 7.6 Hz, 4JHH = 1.5 Hz, Ar
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, tables, and CIF files giving full experimental
procedures, NMR spectra, X-ray crystallographic details, and
details of the calculated structures. The Supporting Information
is available free of charge on the ACS Publications website at
DOI: 10.1021/acs.organomet.5b00155.
H), 6.80 (t, 2H, 3JHH = 7.6 Hz, Ar H), 3.93 (s, 4H, −NCH2), 1.44 (s,
18H, −C(CH3)3) ppm. 13C{1H} NMR (CDCl3, 75 MHz): δ 155.1 (s,
Ar), 136.7 (s, Ar), 127.7 (s, Ar), 126.5 (s, Ar), 123.5 (s, Ar), 119.4 (s,
Ar), 50.9 (s, NCH2), 34.6 (s, C(CH3)3), 29.7 (s, C(CH3)3) ppm.
AUTHOR INFORMATION
Corresponding Author
*E-mail for A.S.V.: veige@chem.ufl.edu.
■
t
Synthesis of [OCH2NHCH2O]WCCR(OtBu) (R = Et (3-Et), Bu
(3-tBuanti)). A representative procedure is given for the synthesis of
3-tBuanti; the same procedure was utilized for the synthesis of 3-Et.
A benzene solution (2 mL) containing 2 (0.096 g, 0.28 mmol, 1
equiv) was added dropwise to a benzene (1 mL) solution of
(tBuO)3WCtBu (0.133 g, 0.280 mmol, 1 equiv). The reaction
mixture was stirred for 0.5 h. All volatiles were evaporated under
vacuum for 1 h. The golden brown powder was triturated with pentane
(×3). After removal of all volatiles, the golden brown powder was
dissolved in a minimal amount of Et2O and filtered. Cooling the
solution to −35 °C precipitated crystals of 3-tBuanti (yield 0.13 g,
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
A.S.V. thanks the University of Florida and the National
Science Foundation for financial support of this project (CHE-
1265993). K.A.A. thanks the University of Florida and the
National Science Foundation (CHE-0821346) for funding the
purchase of X-ray equipment. Computational resources and
support were provided by the University of Florida High-
Performance Computing Center.
1
3
68%). H NMR (C6D6, 500 MHz): δ 7.39 (dd, 2H, JHH = 7.7 Hz,
4JHH = 1.4 Hz, Ar H), 6.80 (t, 2H, 3JHH = 7.7 Hz, Ar H), 6.75 (dd, 2H,
3JHH = 7.7 Hz, JHH = 1.4 Hz, Ar H), 4.68 (dd, 2H, JHH = 12.1 Hz,
4
2
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