Structures and CO2 reactivity of zinc complexes of bis(diisopropyl-) and bis(diphenylphosphino)amines

Article abstract of DOI:10.1021/om500856p

The bis(phosphino)amines (R₂P)NH(PR′₂) (R = R′ = isopropyl; R = R′ = phenyl; R = isopropyl, R′ = phenyl) react with ZnEt₂ to form complexes with two different structural motifs, either the homoleptic monomeric P,P-chelates Zn[N(i-Pr₂P)₂]₂ and Zn[N(i-Pr₂P)(Ph₂P)]₂ or the heteroleptic dimeric Zn₂N₂P₂ heterocycles {EtZn[N(PPh₂)₂]}₂ and {EtZn[N(PPh₂)(i-Pr₂P)]}₂. In two cases, CO₂ reacts with these complexes to give adducts Zn[O₂CP(i-Pr₂)NP(i-Pr₂)]₂ and Zn[O₂CP(i-Pr₂)NPPh₂][Ph₂PN(i-Pr₂P)]ZnEt₂, similar to adducts formed from the reaction of CO₂ with frustrated Lewis pairs (FLPs). In the other two cases, reaction with CO₂ results in cleavage and rearrangement of the N-P bonds to give either N(PPh₂)₃ or Ph₂P(iPr₂P)NPPh₂. The zinc complexes and their CO₂ products were characterized with a combination of single crystal X-ray diffraction and multinuclear NMR spectroscopy.

Full text of DOI:10.1021/om500856p

Article
pubs.acs.org/Organometallics
Structures and CO2 Reactivity of Zinc Complexes of Bis(diisopropyl‑)
and Bis(diphenylphosphino)amines
Diane A. Dickie† and Richard A. Kemp*,†,‡
†Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New Mexico 87131, United States
‡Advanced Materials Laboratory, Sandia National Laboratories, Albuquerque, New Mexico 87106, United States
S
* Supporting Information
ABSTRACT: The bis(phosphino)amines (R2P)NH(PR′2) (R = R′ = isopropyl; R = R′ = phenyl; R = isopropyl, R′ = phenyl)
react with ZnEt2 to form complexes with two different structural motifs, either the homoleptic monomeric P,P-chelates Zn[N(i-
Pr2P)2]2 and Zn[N(i-Pr2P)(Ph2P)]2 or the heteroleptic dimeric Zn2N2P2 heterocycles {EtZn[N(PPh2)2]}2 and {EtZn[N-
(PPh2)(i-Pr2P)]}2. In two cases, CO2 reacts with these complexes to give adducts Zn[O2CP(i-Pr2)NP(i-Pr2)]2 and Zn[O2CP(i-
Pr2)NPPh2][Ph2PN(i-Pr2P)]ZnEt2, similar to adducts formed from the reaction of CO2 with frustrated Lewis pairs (FLPs). In
the other two cases, reaction with CO2 results in cleavage and rearrangement of the N−P bonds to give either N(PPh2)3 or
Ph2P(iPr2P)NPPh2. The zinc complexes and their CO2 products were characterized with a combination of single crystal X-ray
diffraction and multinuclear NMR spectroscopy.
forms an adduct with one P atom of each ligand (two CO2 per
Sn).5 This tin complex was the first structurally characterized
example featuring the monoanionic ligand [(i-Pr2P)2N]−, even
though it is the precursor for the widely used oxidized ligands
[i-Pr2P(X)NP(i-Pr)2]− and {[i-Pr2P(X)]2N}− (X = O, S, Se,
Te).3 A similar discrepancy in the application of the oxidized vs
nonoxidized (Pv vs PIII) ligands is seen in the closely related
[(Ph2P)2N]− system. There are more than 350 structurally
characterized metal complexes of the fully or partially oxidized
monoanionic phenyl-substituted ligand, significantly more than
the nonoxidized form.3
INTRODUCTION
■
The quest for molecules and methods that will facilitate the
chemical, electrochemical or biochemical reduction of carbon
dioxide to rebalance the global carbon cycle is an important and
growing field of research.1 CO2 has long been known to insert
into a variety of highly polarized M−E bonds (E = H, CR3,
NR2, OR), but until the development of the “frustrated Lewis
pair” (FLP) concept, insertion into M−P bonds was rare. The
term FLP was coined by Stephan2 in 2007 to describe
complexes in which a highly Lewis acidic borane and a Lewis
basic phosphine are prevented by sterics from interacting
directly with one another and are thus primed to react with
small molecules. Because CO2 has both nucleophilic and
electrophilic sites, it is an ideal molecule for releasing the
“frustration” of the Lewis pair, and FLPs have become
prominent in CO2 activation. The resulting complexes are
typically described as CO2-adducts and show an average O−C−
O bond angle3 of 126°, closely resembling the geometry
required to use CO2 in a reduced form as a C1 source for fuels
or other value-added products.
Ligands of the general form [(R2P)2N]− have the potential to
be highly versatile because both N and P can act as donors to a
metal, while still maintaining an open Lewis basic site for
reaction with CO2 or other small molecules. Figure 1
summarizes the coordination modes that have been reported
in structurally characterized compounds of this general class
and the approximate number of examples of each.3 Modes B
and D are predominately seen in transition metal complexes,
while main group and rare earth metals generally favor
coordination to the harder nitrogen atom. Herein we describe
the synthesis and structural characterization of Zn complexes of
Similar reactivity, both catalytic and stoichiometric, has since
been reported for a variety of main group and transition metal
Lewis acids, including many that exist as traditional unfrustrated
coordination complexes.4 For example, upon exposure to CO2,
the P,P-chelated Sn(II) complex [(i-Pr2P)2N]2Sn reversibly
Received: August 22, 2014
Published: November 3, 2014
© 2014 American Chemical Society
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= 3.996 Å) suggest that these are extremely weak interactions.8
The N−PPh bonds (Table 1) are shorter (N1−P2 = 1.6908(13)
Å; N2−P4 = 1.6924(13) Å) than the N−PiPr bonds (N1−P1 =
1.7111(13) Å; N2−P3 = 1.7117(13) Å), consistent with the
bond lengths in the homoleptic complexes, which average
1.705(4) Å for 15 and 1.692(2) Å for 2.6a,c The P−N−P angles
in 3 measure 120.76(8) and 120.81(8)°, intermediate between
the angles for 1 [121.2(2) and 121.3(2)°]5 and 2 (118.9(2)°6a
or 118.86°6c).
Figure 1. Coordination modes of structurally characterized metal
complexes of [(Ph2P)2N]−. The number in parentheses indicates the
number of examples found in the CSD.3
Syntheses and structures of Zn complexes. Addition of
diethylzinc to a pentane solution of 1 (Scheme 1) results in the
loss of two equivalents of ethane and formation of [(i-
Pr2P)2N]2Zn 4 as a colorless, crystalline solid in very good
yield. The 31P{1H} spectrum showed a single peak at δ 91.4
ppm, more than 20 ppm downfield from the free ligand 1,
which is found at 68.0 ppm. This chemical shift is strongly
suggestive of P,P-chelation (Figure 1, mode D), the same
coordination pattern seen in the tin analog [(i-Pr2P)2N]2Sn.5
The P,P-chelation mode of 4 was confirmed by single-crystal
X-ray diffraction (Figure 3). In 4, the geometry at Zn is
distorted tetrahedral, with the two ZnP2N rings oriented at an
angle of 88.70(5)° from one another. The P−N bonds are
much shorter and the P−N−P angles more acute in 4 than in
the free ligand 15 (Table 1). This is consistent with an allylic
P−N−P bonding description, with the monoanionic charge
delocalized across the three heteroatoms of each ligand.
In contrast to the virtually unexplored coordination
chemistry of 1, ligand 2 has been shown to exhibit at least
five different coordination modes (Figure 1) with a variety of
main group, transition metal, and lanthanide elements.
Therefore, it was not clear a priori whether the reaction of 2
with diethylzinc would yield an analog of 4 or an entirely
different structural motif. A variety of conditions and metal/
ligand ratios were screened, and in all cases the same product 5
was obtained. Multinuclear NMR spectroscopy showed a single
three different [(R2P)2N]− ligands which were found to
coordinate via modes C and D, and the reactivity of these
complexes with CO2.
RESULTS AND DISCUSSION
■
Ligand synthesis and structure. The syntheses and
structures of the ligands (i-Pr2P)2NH 15 and (Ph2P)2NH 26
have been described elsewhere. The new mixed alkyl/aryl
ligand (i-Pr2P)NH(PPh2) 3 was prepared by the addition of
one equivalent of Ph2PCl to a toluene solution of (i-
Pr2P)NH(SiMe3).7 After stirring at room temperature for 90
min, the solvent and the chlorotrimethylsilane coproduct were
removed under vacuum to leave the white solid 3. The 31P
NMR spectrum of 3 showed two doublets with a 2JPP coupling
of 179 Hz. The signal assigned to the i-Pr2P group appeared at
72.1 ppm while the PPh2 group was found at 40.7 ppm. These
chemical shifts were slightly downfield and upfield, respectively,
from the homoleptic molecules 1 (68.0 ppm) and 2 (44.3
ppm).
X-ray quality single crystals of 3 can be grown from THF
solution at −25 °C. Figure 2 shows the two crystallographically
1
peak at 49 ppm in the 31P spectrum, while the H and 13C
spectra suggested a heteroleptic complex consistent with one
ethyl group remaining bound to the Zn atom. The optimized
conditions for the formation of 5 (Scheme 1) were found to be
an overnight reaction at rt of an equimolar combination of 2
and ZnEt2 in toluene.
Single crystals of 5 were grown from a combination of THF,
pentane, and toluene. X-ray diffraction (Figure 4) revealed that
two of those solvents, toluene and THF, were incorporated into
the crystal lattice, and an additional molecule of THF was
coordinated to Zn1. The ligand acts as an N,P-bridge (Figure 1,
mode C) between two different Zn centers giving a six-
membered boat-like Zn2N2P2 ring. This dimeric heterocyclic
motif has been previously reported. In three of the known
examples,9 the multidentate ligands engage in additional
interactions besides the main heterocycle. That is in contrast
to 5, in which the second P atom of each ligand remains free.
This makes its structure more comparable to the remaining two
literature examples, both of which feature bidentate ligands of
the general form −N(PR2)(SiMe3).10
The Zn−N bonds (Zn1−N1 = 1.993(3); Zn2−N2 =
1.983(3) Å) are near the average for other EtZn-amido
complexes in the CSD (mean Zn−N = 1.995 Å), which
suggests significant covalent character in that bond. An
examination of the N−P bonds, however, indicates that the
NP resonance form shown in Scheme 1 likely also
contributes. As summarized in Table 1, the endocyclic N−P
bonds of 5 are significantly shorter (N1−P2 = 1.659(3); N2−
Figure 2. Structure of 3. Thermal ellipsoids are shown at 50%
probability. For clarity, only N−H hydrogens are shown.
distinct molecules in the asymmetric unit. The phosphorus
atoms exhibit a pyramidal geometry, while both nitrogen atoms
are planar (∑∠N1 = 358.36°; ∑∠N2 = 358.71°). Each amine
hydrogen points directly (∠N1−H1A···P4 = 179.03°; ∠N2−
H2A···P2 = 178.03°) at the lone pair of the other molecule’s
phenyl-substituted phosphorus atom. Given that geometry, it is
tempting to describe these as N−H···P hydrogen bonds, but
the rather long distances involved (N1···P4 = 3.982 Å; N2···P2
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Table 1. Selected Bond Lengths (Å) and Angles (deg) for Ligands 1−3 and Zn Complexes 4−7
Ligands
15
26c
3
N−PiPr
N−PPh
P−N−P
1.704(4)−1.706(4)
1.7111(13)−1.7117(13)
1.6908(13)−1.6924(13)
120.76(8)−120.81(8)
1.692(2)
118.9(2)
121.2(2)−121.3(2)
Zn complexes
4
5
6
7
N−PiPr
N−PPh
1.6403(17)−1.6483(18)
1.6496(19)−1.6532(19)
1.6402(19)−1.6419(19)
1.669(2)−1.674(3) endo
1.700(3)−1.707(3) exo
1.659(3)−1.662(3) endo
1.711(3)−1.719(3) exo
Zn−PiPr
Zn−PPh
Zn−N
2.4184(6)−2.4291(5)
2.3776(6)- 2.3829(6)
2.4344(9)−2.4438(9)
2.4705(11)−2.4276(11)
1.983(3)−1.993(3)
117.32(16)−118.15(17)
CO2 adducts
2.4106(6)−2.4213(6)
1.976(3)−1.981(3)
120.60(16)−121.00(16)
P−N−P
107.87(9)−108.57(9)
107.02(10)−107.07(10)
8
11
N−PiPr/CO2
N−PiPr
1.577(2)−1.582(3)
1.624(3)−1.636(2)
1.579(2)
1.679(2)
N−PPh
1.632(2) endo
1.719(2) exo
Zn−PiPr
Zn−PPh
Zn−O
C−OZn
C−O
2.3387(9)−2.3432(8)
2.4160(7)
2.4108(7)
1.982(2)−2.003(2)
1.256(4)−1.275(4)
1.217(4)−1.224(4)
130.58(17)−132.22(15)
126.6(3)−127.3(3)
2.1245(18)−2.1741(18)
1.288(3)
1.227(3)
P−N−P
O−C−O
119.03(13)−131.82(14)
125.2(2)
Scheme 1. Synthesis of Zn Complexes 4−7
P4 = 1.662(3) Å) than the free ligand 2 (1.692(2) Å), and the
exocyclic N−P bonds (N1−P1 = 1.719(3); N2−P3 = 1.711(3)
Å) are longest of all.
Given the dramatic difference in the coordination modes
observed in 4 and 5, it was of great interest to explore the
behavior of 3 with ZnEt2. For reasons of solubility, the initial
screening reaction was performed in toluene. The reaction was
monitored by 31P{1H} NMR over the course of several days.
The final product, 6, displayed a pair of doublets in the 31P
spectrum at 100.1 and 61.3 ppm, with 2JPP = 300 Hz. The large
downfield shift of the signals relative to the starting ligand 3 was
much more comparable to the change between 1 and 4 than
between 2 and 5, suggesting that 6 had adopted a P,P-chelate
structure (Figure 1, mode D). The 1H and 13C{1H} spectra of 6
obtained after the product had been isolated showed no
evidence of the zinc-ethyl group, which was also consistent with
a P,P-chelate.
Figure 3. Structure of 4. Thermal ellipsoids are shown at 50%
probability. For clarity, H atoms are not shown.
tetrahedron, with an angle of 87.53(5)° between the two
ZnP2N rings. The allylic nature of the anionic PNP fragment is
shown by the shortened, equalized N−P bonds (Table 1). The
fact that they are the same within experimental error is
somewhat surprising, given how different the N−PiPr and N−
P
Ph bond lengths are in the free ligand 3. In contrast to the N−
P bonds, there is a difference in the Zn−P bonds. The Zn−PiPr
bonds in 6 measure 2.3776(6) and 2.3829(6) Å, shorter than
the same bonds in 4 and shorter than the Zn−PPh bonds in
either 5 or 6.
Single crystals of 6 were grown from THF and X-ray
diffraction (Figure 5) confirmed that 6 had the same
coordination mode as 4. The geometry at Zn1 is a distorted
As mentioned above, the initial synthesis of 6, which was
later optimized to a few hours of reflux in toluene, was first
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Figure 6. Structure of 7. Thermal ellipsoids are shown at 50%
probability. For clarity, H atoms and noncoordinated THF solvent are
not shown.
Figure 4. Structure of 5. Thermal ellipsoids are shown at 50%
probability. For clarity, H atoms and noncoordinated solvent
molecules are not shown.
and 7 are identical to those measured in the free ligands 2 and
3, respectively.
Reactivity of Zn complexes with CO2. When CO2 is
bubbled through a pentane solution of 4, a white precipitate 8
is formed within a few minutes (Scheme 2). Two pieces of data
Scheme 2. Reaction of CO2 with 4 To Give Adduct 8
are characteristic of the formation of CO2 adducts−the ν(C
O) stretch in the IR spectrum and the downfield doublet in the
13C{1H} NMR spectrum. The IR spectrum of the Zn product 8
showed a very strong CO stretch at ν =1633 cm−1, close to that
observed for the Sn analog (1626 cm−1),5 and well within the
range of what has been previously reported for P-based
frustrated Lewis pair (FLP) adducts (ν = 1608−1702 cm−1).11
A phosphorus-coupled doublet assigned to coordinated CO2
Figure 5. Structure of 6. Thermal ellipsoids are shown at 50%
probability. For clarity, H atoms are not shown.
1
performed at rt over several days. During the monitoring of that
reaction by 31P{1H} NMR spectroscopy, formation of an
intermediate 7 was observed. Two different signals were
present in the spectrum, but in contrast to the P−P coupled
doublets of 3 and 6, complex 7 gave rise to singlets. While it is
possible to isolate 7 from the reaction leading to 6, higher
yields without traces of 3 or 6 were much easier to achieve from
a rational synthesis based on the reaction of equimolar amounts
of 3 and ZnEt2 in pentane. With that clean product in hand,
further characterization was performed and 7 was found to be
an analog of the heterocyclic dimer 5.
was observed in the 13C{1H} NMR at 168.8 ppm with a JCP
coupling of 90 Hz. Again, this is comparable to both the Sn
analog (168.7 ppm, JCP = 95 Hz)5 and the known FLP-
1
adducts.11,12 Both the H and 13C{1H} spectra of 8 show
1
distinct sets of signals for the isopropyl groups on the CO2-
bound P and the Zn-bound P, with the CO2 set distinctly
broadened. The 31P{1H} NMR spectrum of 8 confirmed the
presence of inequivalent P atoms, displaying two singlets with
no discernible P−P coupling at δ = 31.6 and 45.8 ppm.
In addition to the spectroscopic characterization, single
crystal diffraction studies were performed on 8 (Figure 7). Two
crystallographically independent molecules were found in the
triclinic unit cell. The Zn−P bonds (Table 1) in the adduct 8
are nearly 0.1 Å shorter than in the starting complex 4, and the
P−N bonds are no longer allylic. Instead, there are distinct
shorter (1.577(2)−1.582(2) Å) and longer (1.624(3)−
1.636(2) Å) P−N bonds in each of the four ligands, with the
shorter bonds associated with the CO2-bound P. The C−O
bonds within each CO2 fragment differ, with the O bound to
the metal showing more single bond character and the pendant
O more double bond character. The geometry at Zn remains a
Because the chemical shift assigned to the PiPr in the 31P{1H}
NMR spectrum of 7 is nearly identical to that of 3 at 71.4 and
72.1 ppm, respectively, and the PPh signal had shifted from 40.7
ppm in 3 to 52.2 ppm in 7, we initially thought that the
exocyclic group was the former and the endocyclic the latter.
Single crystal diffraction studies (Figure 6) later revealed that
the opposite was true−it is the PiPr that is bound to Zn, and the
P
Ph that is free. As in 5, the endocyclic N−P bonds (Table 1) in
7 are much shorter than the exocyclic bonds. Despite the
dramatic changes in bond lengths, the P−N−P angles in both 5
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Figure 7. Structure of 8. For clarity, only one of two crystallo-
graphically independent molecules is shown and H atoms were
omitted.
Figure 8. Structure of 10. Thermal ellipsoids are shown at 50%
probability. For clarity, H atoms are not shown. Selected bond lengths
(Å) and angles (deg): P1−P2 = 2.2352(5), P2−N1 = 1.5876(13),
N1−P3 = 1.6778(13), P1−P2−N1 = 120.98(5), P2−N1−P3 =
125.78(8).
distorted tetrahedron in 8 and the six-membered CO2-
containing rings are offset from one another by 88.54(7)° for
the Zn1 molecule and 88.13(7)° for Zn2. The process of going
from four-membered ZnP2N rings in 4 to six-membered rings
in the CO2 adduct 8 allows the P−N−P bond angles to
increase by more than 20° to an average of 131° while the O−
C−O fragments are bent to an average 127°.
and the structural parameters of 10 are very similar to those of
the previously published Ph2P(Ph2P)NPPh2,14a an isomer of 9.
Finally, the reaction of 7 with CO2 was explored. Because 5,
6 and the related group 2 complexes14 had all undergone some
form of N−PPh bond cleavage in the presence of CO2, and
because Zn2N2P2 heterocyclic dimers have also been shown to
disproportionate under similar conditions,10b a complex
mixture of products was anticipated from this reaction (Scheme
3). Upon bubbling CO2 through a pentane solution of 7, a
As was described above, there are only a few six-membered
Zn2N2P2 heterocycles similar to 5 in the literature. One of
these, based on the ligand -N[P(i-Pr2)(SiMe3)], has been
shown to form an adduct very similar to 8 although the
mechanism for its formation is somewhat more complicated
than simple insertion into a Zn−P bond.10b Therefore, it was
expected that the reaction of 5 with CO2 would also give a
CO2-inserted product, but likely not cleanly. It was somewhat
surprising, therefore, to find that upon bubbling carbon dioxide
through a toluene solution of 5, only a single phosphorus-
containing product was observed by 31P{1H} NMR at 59 ppm.
The IR spectrum of the isolated powder, 9, did not show any
stretches characteristic of CO2 insertion. Single crystals of 9
were grown and were found to be the tertiary amine
N(PPh2)3.13 We have been unable to determine the fact of
Zn in this reaction. We have previously reported the cleavage
and rearrangement of N−P bonds in the presence of CO2 to
give 9 and/or its isomer Ph2P(Ph2P)NPPh2,14 and similar
oxidative scrambling of the [N(PPh2)2]− ligand under a variety
of conditions has been reported by others.15
Despite its structural similarity with 4, solutions of 6 in either
pentane or toluene showed no reaction with CO2 under the
time, temperature and pressure conditions used for 4 and 5.
Upon switching to the polar solvent THF, however,
spectroscopic evidence for a reaction was observed. A strong
stretch in the IR spectrum grew in at 1639 cm−1, while the
31P{1H} NMR collapsed to two broad singlets at 34.9 and 21.3
ppm. These data are consistent with the formation of a CO2-
adduct similar to 8, but in this case it was not isolable. Instead,
the product that was ultimately obtained from the reaction was
more closely related to 9. It was possible to identify 10 as
Ph2P(iPr2P)NPPh2 by both 31P{1H} NMR and single crystal X-
ray diffraction (Figure 8). The central isopropyl-substituted
phosphorus atom has the most downfield chemical shift at 44.5
ppm and appears as a doublet of doublets with couplings of 267
Hz to its directly bound neighbor at −29.7 ppm, and 82 Hz to
the N-bound phosphorus atom at 39.7 ppm. The NMR pattern
Scheme 3. Reaction of CO2 with 7 to give 11
white precipitate 11 was obtained. The stretch at 1634 cm−1 in
the IR spectrum was strong evidence that 11 was a CO2
insertion product. Unfortunately, no signal for this moiety was
observed in the 13C NMR spectrum. The alkyl and aryl regions
1
of the H and 13C{1H} NMR spectra were complicated, but
suggested that the ethyl group was still present on zinc. These
spectra also indicated that there was little symmetry in the
product molecule. This was consistent with the 31P{1H} NMR
spectrum that showed four different phosphorus environments.
In order to elucidate the structure of 11, single crystals were
grown by vapor diffusion of pentane into a concentrated
toluene solution. The structure (Figure 9) was consistent with
the unsymmetric molecule suggested by the NMR data. One of
the two ligands present in the starting material 7 retained its
original coordination mode, binding to one zinc atom of the
dimer through nitrogen and to the other through isopropyl-
substituted phosphorus. The CO2 molecule was found to have
inserted itself into the Zn−PiPr bond of the other ligand, with
the bound oxygen bridging both Zn atoms. The retained ethyl
groups, and rearrangement of the CO2-bound ligand to
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Anhydrous solvents were stored in the glovebox over 4 Å molecular
sieves prior to use. 1H and 13C{1H} spectra were referenced to residual
solvent downfield of TMS. 31P{1H} spectra were referenced to
external 85% H3PO4. IR spectra were recorded as mulls or thin films
on KBr windows. Single-crystal X-ray diffraction studies were
performed on a Bruker Kappa Apex II CCD diffractometer. Crystals
were coated in Paratone-N oil and mounted on either a Cryoloop or
MiTeGen MicroLoop. The Bruker Apex2 software suite was used for
data collection, structure solution and refinement. Relevant parameters
are summarized in Tables S1 and S2 (Supporting Information).
HN(i-Pr2P)(Ph2P) (3). A solution of chlorodiphenylphosphine (1.07
g, 4.87 mmol) in ca. 15 mL toluene was added dropwise to a solution
of N-trimethylsilyl-N-diisopropylphosphinoamine7 (1.00 g, 4.87
mmol) in ca. 10 mL toluene. The resulting slightly cloudy, pale
yellow solution was stirred at rt for 90 min, then the solvent was
removed under vacuum to give a white powder. Yield =1.41 g (91%),
mp =101−102 °C. X-ray quality single crystals were grown from a
1
concentrated THF solution. H NMR (C6D6, 300 MHz) δ 0.93 (dd,
3JHP = 10.9 Hz, 3JHH = 6.9 Hz, 6H, CH(CH3)2), 0.94 (dd, 3JPH = 14.8
Figure 9. Structure of 11. Thermal ellipsoids are shown at 50%
probability. For clarity, H atoms are not shown.
3
2
Hz, JHH = 6.9 Hz, 6H, CH(CH3)2), 1.50 (sept of d, JHP = 2.4 Hz,
3JHH = 6.9 Hz, 2H, CH(CH3)2), 2.17 (dd, 2JHP = 8.6 Hz, 2JHP = 5.5 Hz,
1H, NH), 7.02−7.13 (overlapping m, 6H, aryl H), 7.40−7.46
(overlapping m, 4H, aryl H) ppm. 13C{1H} NMR (CD2Cl2, 75
coordinate through the phenyl-substituted P instead of through
N, completed the coordination sphere of the Zn atoms.
The Zn−O bonds in 11 are longer (2.1245(18)−2.1741(18)
Å) than in 8 (1.982(2)−2.003(2) Å), as is expected for a
bridging O atom, while the C−O and CO bonds of 8 and 11
are the same within experimental error. Upon binding CO2, the
N−P bond decreases by 0.1 Å compared to 7 and the
“unreacted” side of 11. Instead, it is in the middle of the range
of bond lengths measured for the N−P bonds in 8. Nearly as
big a difference is observed with the endo- and exocyclic N−P
bonds, with the exo being slightly longer (1.719(2) Å) than in 7
(1.700(3)−1.707(3) Å) and the endo being shorter (1.632(2)
Å) than in 6. Taken together, this suggests that the P1−N1−P2
fragment has more allylic character than in 8. It is not surprising
that a second molecule of CO2 does not insert into the
remaining Zn−PiPr bond because the newly formed five-
membered Zn−O−Zn−N−P ring should be stable and
unstrained.
2
2
MHz) δ 17.1 (d, JPC = 8.4 Hz, CH(CH3)2), 18.7 (d, JPC = 19.9 Hz,
1
3
CH(CH3)2), 27.2 (dd, JPC = 14.5 Hz, JPC = 8.3 Hz, CH(CH3)2),
128.2 (d, JPC = 6.5 Hz, meta-C), 128.7 (s, para-C), 130.9 (d, JPC
3
2
=
1
3
21.1 Hz, ortho-C), 143.7 (dd, JPC = 15.7 Hz, JPC = 7.7 Hz, ipso-C)
ppm. 31P{1H} NMR (C6D6, 121 MHz) δ 40.7 (d, JPP = 179 Hz,
2
2
PPh2), 72.1 (d, JPP = 179 Hz, P(i-Pr)2 ppm. Anal. Calcd for
C18H25NP2: C, 68.13; H, 7.94; N, 4.41. Found: C, 68.21; H, 8.45; N,
4.27.
Zn[N(i-Pr2P)2]2 (4). Diethylzinc (3.0 mL, 3.0 mmol, 1.0 M in
hexanes) was added dropwise to a solution of 1 (1.50 g, 6.0 mmol) in
ca. 15 mL anhydrous pentane. The colorless solution was allowed to
stir overnight at rt before ca. half the solvent was removed under
vacuum. The concentrated solution was cooled to −25 °C. Within 24
h, colorless crystals of 4 were formed and were isolated by decanting
the supernatant solution. Yield =1.15 g (68%), mp >200 °C. 1H NMR
(C6D6, 500 MHz) δ 1.17−1.27 (overlapping doublets, 48H,
PCH(CH3)2), 1.89−1.95 (br sept, 8H, PCH(CH3)2) ppm. 13C{31P,
1H} NMR (C6D6, 125 MHz) δ 17.9 (s, PCH(CH3)2), 19.1 (s,
PCH(CH3)2), 29.4 (s, PCH(CH3)2) ppm. 31P{1H} NMR (C6D6, 121
MHz) δ 91.4 ppm. Anal. Calcd for C24H56N2P4Zn: C, 51.29; H, 10.04;
N, 4.98. Found: C, 51.60; H, 9.67; N, 4.80.
CONCLUSIONS
■
{EtZn[N(PPh2)2]}2 (5). ZnEt2 (3.4 mL, 3.4 mmol, 1.0 M in hexanes)
was added slowly to a suspension of 2 (1.10 g, 2.85 mmol) in ca. 20
mL toluene. During the addition, the suspension became a colorless
solution. The solution was allowed to stir overnight at rt before the
solvent was removed under vacuum to give the white powder 5. Yield
=1.29 g (98%), mp =124 °C (dec). X-ray quality single crystals were
grown from a concentrated 1:1:5 solution of toluene, THF and
We have prepared and characterized four complexes from the
reaction of ZnEt2 with (i-Pr2P)2NH, (Ph2P)2NH and (i-
Pr2P)NH(PPh2). As the substituent on phosphorus is changed
from isopropyl to phenyl, the coordination mode of the ligand
also changes. Coordination through phosphorus is favored for
alkyl substituents, while aryl groups also permit coordination
through N. This difference in donor ability is supported by
recent calculations by Chivers et al. on the nucleophilicity of
(R2P)2NH ligands.16 Two different structural motifs were
obtained, either a P,P-chelated, disubstituted monomer, or a
Zn2N2P2 monosubstituted heterocycle. The all-isopropyl
chelate and the mixed iPr/Ph heterocycle both react with
CO2 via insertion into a P−Zn bond. In contrast, the mixed
iPr/Ph chelate and the all-phenyl heterocycle undergo N−P
bond cleavage in the presence of CO2 to form Ph2P(iPr2P)-
NPPh2 and N(PPh2)3, respectively, as had been previously seen
for group 2 complexes of [(Ph2P)2N]− under similar
conditions.14
1
pentane. H NMR (C6D6, 300 MHz) δ 0.32 (br, 4H, ZnCH2CH3),
1.05 (t, 3JHH = 8.1 Hz, 6H, ZnCH2CH3), 6.94−7.05 (br m, 24H, meta/
para C6H5), 7.46−7.58 (br m, 16 H ortho-C6H5) ppm. 13C{1H} NMR
(THF, 75 MHz) δ 7.0 (br, ZnCH2CH3), 12.0 (s, ZnCH2CH3), 128.1
2
(br s, meta-C), 128.8 (s, para-C), 132.8 (d, JCP = 18 Hz, ortho-C),
141.8 (s, ipso-C) ppm. 31P{1H} NMR (C6D6, 121 MHz) δ 49.7 ppm.
Anal. Calcd for C52H50N2P4Zn2: C, 65.22; H, 5.26; N, 2.93. Found: C,
65.14; H, 6.03; N, 2.74.
Zn[N(i-Pr2P)(Ph2P)]2 (6). ZnEt2 (1.6 mL, 1.6 mmol, 1.0 M in
hexanes) was added to a solution of 3 (1.00 g, 3.2 mmol) in ca. 10 mL
toluene. The colorless solution was heated to reflux under argon for 3
h. After cooling to rt, the solvent was removed under vacuum to give a
white residue that was suspended in pentane and filtered. Yield =0.67 g
(61%), mp =151−152 °C. X-ray quality single crystals were grown
1
from a 1:1 mixture of pentane and THF. H NMR (CD2Cl2, 300
MHz) δ 0.99 (br dd, 6H, CH(CH3)2), 1.16 (dd, 3JHH = 7.2 Hz, 3JPH
14.8 Hz, 6H, CH(CH3)2), 2.02 (sept of d, JHH = 7.2 Hz, JPH = 14.0
Hz, 2H, CH(CH3)2), 7.31−7.35 (br m, 6H, meta/para C6H5), 7.47−
7.54 (br m, 4H, ortho-C6H5) ppm. 13C{1H} NMR (CD2Cl2, 75 MHz)
=
EXPERIMENTAL SECTION
General experimental. All manipulations were carried out in an
argon-filled glovebox or by using standard Schlenk techniques. Ligands
15 and 26b were prepared according to literature procedures.
■
3
2
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Organometallics
Article
1
1
δ 17.0 (s, CH(CH3)2), 18.0−19.2 (vm, CH(CH3)2), 27.9 (vt, JPC
=
concentrated toluene solution. H NMR (C6D6, 300 MHz) δ 0.42−
0.54 (br m, 4H, ZnCH2CH3), 0.81−1.42 (overlapping dd, 24H,
18.3 Hz, CH(CH3)2), 128.0 (d, 3JCP = 9.2 Hz, meta-C), 128.8 (s, para-
C), 130.3 (d, 2JCP = 13.9 Hz, ortho-C), 142.1 (dd, 1JPC = 26.1 Hz, 3JPC
= 19.0 Hz, ipso-C) ppm. 31P{1H} NMR (CD2Cl2, 121 MHz) δ 62.1
(d, 2JPP = 298 Hz, PPh2), 101.3 (d, 2JPP = 298 Hz, P(iPr)2) ppm. Anal.
Calcd for C36H48N2P4Zn: C, 61.94; H, 6.93; N, 4.01. Found: C, 62.54;
H, 7.22; N, 3.86.
3
CH(CH3)2), 1.53−1.66 (br, 3H, O2CZnCH2CH3), 1.84 (t, JHH = 8
Hz, 3H, ZnCH2CH3), 2.28−2.33 (br, 3 H, CH(CH3)2), 2.76−2.81
(br, 1H, CH(CH3)2), 6.89−7.30 (overlapping m, 12H, C6H5), 7.78−
7.92 (overlapping m, 8H, C6H5) ppm. 13C{1H} NMR (C6D6, 75
MHz) δ −1.8 (br s, ZnCH2) 13.1 (s, CH3), 13.9 (s, CH3), 15.4 (s,
CH3), 16.4 (s, CH3), 24.6 (br s, CH(CH3)2), 25.3 (br s, CH(CH3)2),
128.7 (s, aryl-C), 131.7 (s, aryl-C), 131.9 s, aryl-C), 143.1 (s, aryl-C)
ppm. CO2 signal not observed. 31P{1H} NMR (C6D6, 121 MHz) δ
23.2 (dd, 2JPP = 94.0 Hz, 2JPP = 31.2 Hz, ZnPPh2), 37.1 (d, 2JPP = 31.2
Hz, O2CP(i-Pr)2), 53.2 (d, J = 6.2 Hz, PPh2), 66.0 (d, 2JPP = 94.0 Hz,
P(i-Pr)2) ppm. IR (nujol mull) ν 1634 (s) cm−1. Anal. Calcd for
C41H58N2O2P4Zn2: C, 56.89; H, 6.75; N, 3.24. Found: C, 56.59; H,
7.02; N, 2.51.
{EtZn[N(PPh2)(i-Pr2P)]}2 (7). A suspension of 3 (0.50 g, 1.58 mmol)
in ca. 20 mL anhydrous pentane was added to a solution of ZnEt2 (1.6
mL, 1.6 mmol, 1.0 M in hexanes) in ca. 10 mL pentane. The white
suspension was allowed to stir overnight at rt before the solvent was
removed under vacuum to give the white powder 7. Yield =0.59 g
(90%), mp =147−150 °C. X-ray quality single crystals were grown
1
from a concentrated pentane/THF solution. H NMR (C6D6, 300
MHz) δ 0.35 (q, 3JHH = 8.1 Hz, 4H, ZnCH2CH3), 1.02 (dd, 3JHH = 6.9
Hz, 3JHP = 17 Hz, 12H, CH(CH3)2), 1.03 (dd, 3JHH = 6.9 Hz, 3JHP = 12
3
3
4
Hz, 12H, CH(CH3)2), 1.30 (tt, JHH = 8.1 Hz, JHP = JHP = 1.5 Hz,
6H, ZnCH2CH3), 2.56 (br sept, 4H, CH(CH3)2), 7.02−7.19 (br m,
12H, meta/para C6H5), 7.70−7.75 (m, 8H, ortho-C6H5) ppm.
ASSOCIATED CONTENT
* Supporting Information
CIF files for 3−8, 10, and 11. Crystallographic data tables and
figures showing cocrystallized solvent in 5 and 7 and both
independent molecules of 8. This material is available free of
charge via the Internet at http://pubs.acs.org. The CIF files
have also been deposited at the CCDC (1017724−1017731).
■
S
13C{1H} NMR (CD2Cl2, 75 MHz) δ 9.5 (d, JPC = 58.8 Hz,
2
2
ZnCH2CH3), 11.7 (s, ZnCH2CH3), 16.5 (d, JPC = 4.6 Hz,
1
CH(CH3)2), 17.9−18.9 (vm, CH(CH3)2), 25.4 (vt, JPC = 11.4 Hz,
CH(CH3)2), 128.0 (d, 3JPC = 5.3 Hz, meta-C), 128.1 (s, para-C), 131.7
2
1
(d, JPC = 20.8 Hz, ortho-C), 145.5 (d, JPC = 19.0 Hz, ipso-C) ppm.
31P{1H} NMR (C6D6, 121 MHz) δ 52.3 (s, PPh2), 71.4 (s, P(i-Pr)2)
ppm. Anal. Calcd for C40H58N2P4Zn2: C, 58.48; H, 7.12; N, 3.41.
Found: C, 57.45; H, 7.10; N, 3.73.
AUTHOR INFORMATION
■
Corresponding Author
Zn[O2CP(i-Pr2)NP(i-Pr2)]2 (8). Carbon dioxide was bubbled through
a solution of 3 (0.50 g, 0.89 mmol) in ca. 10 mL pentane for 15 min.
The white precipitate that formed was separated by filtration through a
glass frit. Yield =0.46 g (79%), mp =98−99 °C. X-ray quality crystals
were grown at −25 °C from the concentrated supernatant solution. 1H
NMR (C6D6, 300 MHz) δ 1.13 (br, 24H, O2CPCH(CH3)2), 1.18 (d,
* E-mail: rakemp@unm.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
3JHH = 7.2 Hz, 12H, PCH(CH3)2), 1.23 (d, JHH = 7.2 Hz, 12H,
3
■
This work was financially supported by the National Science
Foundation (Grant CHE12-13529) and by the Laboratory
Directed Research and Development (LDRD) program at
Sandia National Laboratories (LDRDs 14938 and 151300).
D.A.D. was financially supported by the Natural Sciences and
Engineering Research Council (NSERC) of Canada by means
of a Postdoctoral Fellowship. The Bruker X-ray diffractometer
was purchased via a National Science Foundation CRIF:MU
award to the University of New Mexico (CHE04-43580), and
the NMR spectrometers were upgraded via grants from the
NSF (CHE08-40523 and CHE09-46690). Sandia is a multi-
program laboratory operated by Sandia Corporation, a
Lockheed Martin Company, for the United States Department
of Energy under Contract No. DE-AC04-94AL85000.
PCH(CH3)2), 1.68 (br, 2H, O2CPCH(CH3)2), 1.92 (br, 2H,
O2CPCH(CH3)2), 2.13−2.26 (overlapping sept, 4H, PCH(CH3)2)
ppm. 13C{1H} NMR (C6D6, 125 MHz) δ 15.9−19.3 (overlapping br,
CH3), 25.6−28.9 (overlapping br, CH), 168.8 (d, 1JCP = 90 Hz, O2CP)
ppm. 31P{1H} NMR (C6D6, 121 MHz) δ 31.6 (s, O2CP(i-Pr)2), 45.8
(s, P(i-Pr)2) ppm. IR (nujol mull) ν 1633 (s) cm−1. Anal. Calcd for
C26H56N2O4P4Zn: C, 48.04; H, 8.68; N, 4.31. Found: C, 48.09; H,
8.67; N, 4.15.
N(PPh2)3 (9). Carbon dioxide was bubbled through a solution of 5
(0.20 g, 0.21 mmol) in ca. 10 mL toluene for 15 min. The white
precipitate 9 was isolated by filtration. Yield =0.07 g (68%), mp =195−
197 °C. X-ray quality single crystals were grown from a concentrated
toluene solution and were found to be identical to the previously
reported N(PPh2)3.13 31P{1H} NMR (toluene, 121 MHz) δ 59 ppm.
Ph2P(iPr2P)NPPh2 (10). Carbon dioxide was bubbled through a
solution of 6 (0.25 g, 0.36 mmol) in ca. 10 mL THF for 15 min.
Solvent was removed under vacuum to give a colorless oil that was
then dissolved in 2 mL of a 1:1 mixture of THF and pentane. The
solution was cooled to −25 °C and colorless crystals of 10 were
isolated after several days by decanting the supernatant solution. mp
=121 °C. 1H NMR (CD2Cl2, 300 MHz) δ 1.13 (dd, 2JPH = 16 Hz, 3JHH
= 7.2 Hz, 6H, CH(CH3)2), 1.23 (dd, 2JPH = 16 Hz, 3JHH = 7.2 Hz, 6H,
CH(CH3)2), 2.45 (br sept of d, 3JHH = 7.2 Hz, 2H, CH(CH3)2), 7.25−
7.85 (br m, 20H, C6H5) ppm. 13C{1H} NMR (CD2Cl2, 75 MHz) δ
17.4 (s, CH3), 29.9 (d, 1JCP = 49 Hz, CH), 127.2 (s, aryl-C), 127.6 (d,
JCP = 6 Hz, aryl-C) 128.4 (d, JCP = 8 Hz, aryl-C), 129.6 (d, JCP = 12 Hz,
aryl-C), 129.9 (s, aryl-C), 135.9 (d, JCP = 20 Hz, aryl-C) ppm (ipso C
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