University of Groningen
The synthesis and exchange chemistry of frustrated Lewis pair–nitrous oxide
complexes
Neu, Rebecca C.; Otten, Edwin; Lough, Alan; Stephan, Douglas W.
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Chemical Science
DOI:
10.1039/c0sc00398k
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Citation for published version (APA):
Neu, R. C., Otten, E., Lough, A., & Stephan, D. W. (2011). The synthesis and exchange chemistry of
frustrated Lewis pair–nitrous oxide complexes. Chemical Science, 2(1), 170-176. DOI:
10.1039/c0sc00398k
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The synthesis and exchange chemistry of frustrated Lewis pair–nitrous oxide
complexes†
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Rebecca C. Neu, Edwin Otten, Alan Lough and Douglas W. Stephan*
Received 29th July 2010, Accepted 19th August 2010
DOI: 10.1039/c0sc00398k
Facile activation of nitrous oxide is achieved using a series of fluoroarylboranes, B(C6F5)3, PhB(C6F5)2,
MesB(C6F5)2, (C6F5)2BOC6F5, B(C6F4-p-H)3, B(C6H4-p-F)3 and 1,4-(C6F5)2BC6F4B(C6F5)2 in the
presence of the basic, bulky phosphine tBu3P. Room temperature reaction yields mono- and biszwitterionic species of the form tBu3P(N2O)B(C6F5)2R (R ¼ C6F5 1, Ph 2, Mes 3, OC6F5 4),
t
Bu3P(N2O)BR3 (R ¼ C6F4-p-H 5, C6H4-p-F 6) and tBu3P(N2O)B(C6F5)2C6F4(C6F5)2B(ON2)PtBu3 7.
N2O activation is similarly achieved using Cy3P and B(C6F4-p-H)3 yielding the zwitterionic species
Cy3P(N2O)B(C6F4-p-H)3 8. Reaction of 6 with [Ph3C][B(C6F5)4] results in facile transfer of the robust
t
Bu3P(N2O) fragment to the stronger Lewis acid Ph3C+ generating [tBu3P(N2O)CPh3][B(C6F5)4] 10.
Similarly exchange reactions with titanocene and zirconocene cations generate transition metal and
phosphine stabilized nitrous oxide salts, of the form [tBu3P(N2O)MCp2Me][MeB(C6F5)3], (M ¼ Zr 11,
Ti 12). The alkoxy zirconocene cation [Cp*2Zr(OMe)]+ forms an FLP in the presence of tBu3P which
reacts with N2O providing a direct synthetic route to the corresponding salt
[tBu3P(N2O)ZrCp*2(OMe)][B(C6F5)4] 13. Kinetic studies of the self-exchange reaction of
t
Bu3P(N2O)B(C6H4-p-F)3 with B(C6H4-p-F)3 were carried out acquiring information regarding the
mechanism of exchange.
Introduction
The issues of ozone depletion and climate change have garnered
a considerable amount of attention over the past century due to
their apparent negative impact on the environment.1 Anthropogenic emissions of potent greenhouse gases, such as carbon
dioxide (CO2), methane (CH4) and nitrous oxide (N2O), continue
to rise and contribute to the phenomenon of global warming and
to the destruction of the stratospheric ozone layer.1,2 Considerable attention has focused on CO2 and its contribution to
global warming, while nitrous oxide (N2O) has remained less
recognized despite being roughly 300 times more potent a greenhouse gas than CO2.2 N2O is inherently stable and can persist for
upwards of 150 years in the stratosphere.3 Recent reports indicate
that levels of N2O have risen steadily since industrialization from
roughly 270 to 319 ppb, as of 2005, and continue to rise annually.4
In fact, anthropogenic N2O has recently been identified as the
largest global ozone depleting gas emission.1
Department of Chemistry, University of Toronto, 80 St. George Street,
Toronto, Ontario, Canada M5S 3H6. E-mail: [email protected]
† Electronic
supplementary
information
(ESI)
available:
Crystallographic data (CIF format) for 1, 2, 5, 7, 8–11 and 13,
synthesis and spectroscopic and analytical data for all new compounds.
NMR spectra of the product from reaction of tBu3P and BPh3 with
N2O, and details for the 19F EXSY NMR analysis. CCDC reference
numbers 768908 and 786852–786857. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c0sc00398k
170 | Chem. Sci., 2011, 2, 170–176
In light of the effects of this greenhouse gas, methods to
sequester and subsequently break down N2O into less harmful
components, such as N2 and H2O, are desirable. In nature, N2O
is transformed by metalloenzymes bearing an active site composed of a tetranuclear copper cluster interconnected by a sulfide
ligand (Fig. 1).5 The metalloenzyme nitrous oxide reductase is an
essential component of the microbial denitrification process,
which is a key component of the nitrogen cycle.3,5a,6 This enzyme
Fig. 1 Resonance structures of N2O (A), Taube’s Ru–N2O complex
(B),8 Hillhouse’s N2O insertion product into Cp*2Ti(C2Ph2) (C),10 and
the Cu4S active site in nitrous oxide reductase (D).11
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in bacteria is responsible for catalyzing the transformation of
N2O to N2 and H2O and is key to the destruction of N2O produced by the agricultural sector.4
In synthetic chemistry, N2O has been shown to be a strong
oxidant yet also a kinetically stable species.7 In coordination
chemistry, N2O has been identified as an inherently poor ligand
due to its inability to act as either a good s-donor or p-acceptor
(the most important resonance structures are shown in Fig. 1).7
The first example of a transition metal bound N2O complex,
[Ru(NH3)5N2O]+, was proposed by Armor and Taube in 1969
(Fig. 1).8 Although this species was never crystallographically
confirmed, a number of both computational and spectroscopic
studies have since supported this formulation.9 Interestingly, few
transition metal–N2O species have been reported in the literature
since Armor and Taube’s discovery in the late sixties,7 and only
recently has an example of a species that contains N2O bound
between a phosphine and a Zn center been crystallographically
characterized.12 N2O is known to undergo reactions at transition
metal centers including oxygen insertion into metal–carbon
(Fig. 1) and metal–hydride bonds with concomitant N2 liberation10,13 and the oxidation of low-valent metal centers.14
Alternatively N–N scission can be achieved in the presence of
metal centers15 while hydrogenation of N2O yields N2 and H2O.16
Very recently, nickel(II) carbene complexes have been shown to
react with N2O via 1,3-dipolar cycloaddition to result in C]O
bond formation with liberation of N2.17 While it is suspected that
these reactions proceed through an initial N2O–metal adduct,
such claims are yet to be unequivocally substantiated.
Recent investigations of the interaction of bulky phosphines
with boranes have given rise to the concept of frustrated Lewis
pairs (FLPs).18 These systems encompass sterically hindered
acids and bases that are precluded from classical Lewis acid–base
adduct formation. The resulting unquenched acidity and basicity
offer the possibility for further reactivity with a variety of small
molecules, such as H2, olefins, dienes, alkynes and carbon dioxide. Such activations have been recently reviewed.18c Herein, the
interaction of FLPs with N2O is shown to result in binding of the
N2O molecule between the Lewis acidic and basic components in
a P–N–N–O–B mode. In addition, the use of Lewis acid
exchange chemistry is explored as a route to new N2O compounds. The initial results of this work were communicated
previously.19
Scheme 2 Reaction of the C6F4-linked bisborane with two equivalents
of tBu3P and N2O.
Lewis acidic boranes R2BR0 with an atmosphere of N2O in a
bromobenzene solution resulted in the formation of novel zwitterionic species over a 12 h period at ambient temperatures
(Scheme 1).
Precipitation with hexanes or pentane afforded the zwitterionic species tBu3P(N2O)BR2R0 (R ¼ R0 ¼ C6F5 (1); R ¼ C6F5,
R0 ¼ Ph (2); R ¼ C6F5, R0 ¼ Mes (3); R ¼ C6F5, R0 ¼ OC6F5 (4);
R ¼ R0 ¼ C6F4-p-H (5); R ¼ R0 ¼ C6H4-p-F (6)) as analytically
pure white microcrystalline solids in yields ranging from 76–91%.
In an analogous manner, reaction of the linked bisborane
(C6F5)2B(C6F4)B(C6F5)2 with two equivalents of tBu3P under an
atmosphere of N2O gave the bis-zwitterionic compound tBu3P(N2O)B(C6F5)2C6F4(C6F5)2B(ON2)PtBu3 (7) in good isolated
yield (Scheme 2).
Conversely, attempts were made to isolate the corresponding
N2O compound from reactions of tBu3P with the weak Lewis
acid BPh3 and N2O. Although the desired product was formed in
low yield, the white powder isolated upon precipitation with
hexanes was 90% pure by NMR spectroscopy (Fig. S1) and
attempts to further purify the material invariably led to
increasing amounts of decomposition. This suggests that a
threshold of Lewis acidity is required to generate stable compounds of this type. NMR spectroscopic analyses of compounds
1–7 in CD2Cl2 showed a single resonance in the 31P{1H} NMR
spectra between 64 and 69 ppm. 11B{1H} NMR spectra
Results and discussion
Complexation of N2O by FLPs
Treatment of frustrated Lewis pairs consisting of an equimolar
mixture of the sterically demanding Lewis base tBu3P and the
Scheme 1 Synthetic method for the generation of 1–6.
This journal is ª The Royal Society of Chemistry 2011
Fig. 2
31
P{1H} and 15N NMR spectra for 4-15N.
Chem. Sci., 2011, 2, 170–176 | 171
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Table 1
15
N NMR chemical shifts and coupling constants for compounds 1-15N to 11-15N and 13-15N
dN(1) (ppm)
dN(2) (ppm)
1
1
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2
JN–N (Hz)
JP–N (Hz)
JP–N (Hz)
1
2
3
4
5
6
7
8
10
11
13
381.7
566.6
377.0
577.7
375.3
574.2
389.4
572.5
367.6
588.8
381.3
571.0
382.3
572.1
376.1
573.0
412.3
548.4
398.5
587.6
386.3
591.0
15.6
58.7
19.6
16.0
59.5
19.1
15.1
59.1
19.6
15.8
58.6
19.7
16.4
59.7
19.1
15.2
58.8
19.9
15.5
58.8
19.6
15.6
52.5
22.2
14.7
58.2
20.5
17.1
60.6
15.8
17.4
61.6
16.1
demonstrated a sharp singlet between 0.67 and 6.69 ppm. For
species containing C6F5 groups, a significant narrowing of the
meta–para gap in comparison to the free boranes was observed in
the 19F NMR spectra, with D(d(m-F)–d(p-F)) around 5 ppm.
These data are consistent with 4-coordinate boron centers.20
Compound 4 bears two distinct C6F5 environments (C6F5
and OC6F5) both of which display meta–para gap narrowing
upon the generation of the anionic boron center. Upfield
19
F NMR shifts are noted for 5 (134.27/143.10 ppm) and
6 (120.87 ppm) relative to the free boranes (129.60/138.74
and 106.00 ppm, respectively). The synthesis of the 15N2O
isotopologues 1-15N–7-15N by reaction of the FLPs with 15N15NO
further supported the generation of the P–N–N–O–B species.
The observation of two doublets of doublets in the 15N NMR
spectra at roughly 377 and 577 ppm for the terminal and central
N atom, respectively, is indicative of two inequivalent and distinct nitrogen environments. 1JN–N coupling constants were
found to be approximately 16 Hz, while 1JN–P and 2JN–P were
determined to be approximately 60 and 20 Hz, respectively.
Comparison of the 15N spectroscopic data of the bound N2O
fragment with free N2O (135 and 218 ppm, 1JN–N ¼ 8.1 Hz)
indicates that there is a significant perturbation of the N2O
fragment upon complexation by the borane and phosphine
moieties (Fig. 2). A comparison of the 15N NMR shifts and
N–N/N–P coupling constants for compounds 1–7 (Table 1)
Fig. 3 POV-ray depiction of the molecular structure of 7. Hydrogen
atoms have been omitted for clarity.
172 | Chem. Sci., 2011, 2, 170–176
Table 2 Metrical parameters for 1, 2, 5, 7 and 8a
P–N(1)
N(1)–N(2)
N(2)–O
O–B
1
2
5
7
8
1.7087(12)
1.2573(17)
1.3363(15)
1.5428(18)
1.7107(6)
1.2602(8)
1.3270(8)
1.5475(9)
1.7092(16)
1.255(2)
1.327(2)
1.533(2)
1.6905(16)
1.253(2)
1.3198(19)
1.536(2)
1.7146(15)
1.260(2)
1.3351(18)
1.551(1)
P–N(1)–N(2) 117.04(10) 112.85(5) 115.32(13) 116.48(14) 111.10(12)
N(1)–N(2)–O 109.11(11) 111.68(6) 109.49(14) 109.81(15) 111.13(14)
N(2)–O–B
114.43(10) 111.61(5) 114.86(14) 111.26(13) 110.32(13)
a
angles in degrees ( ).
Bond lengths in angstroms (A),
shows no significant difference upon the reduction of the Lewis
acidity of the borane fragment.
With the exception of 6, all compounds could be recrystallized
by diffusion of pentane or cyclohexane into a CH2Cl2 or C6H5Br
solution at room temperature. Crystal structure determinations
were carried out for these compounds, unambiguously confirming their formulation, although the quality of the data for compounds 3 and 4 was quite poor and only served to establish
connectivities. The molecular structure of 7 is shown in Fig. 3.
Comparison of the relevant metrical parameters (Table 2) reveals
no significant variation of the bound N2O fragment regardless of
the nature of the borane.
From the data presented above, it is clear that N2O complexes
can be made from combinations of tBu3P with various boron
containing Lewis acids demonstrating that a wide range of Lewis
acidities is tolerated. It was thus of interest to examine a series of
Lewis basic FLP partners for their ability to form analogous
N2O complexes. Both phosphorus (Mes3P, (o-tolyl)3P, HPtBu2)
and nitrogen bases (2,2,6,6-tetramethylpiperidine, 2,6-lutidine,
N-benzylidine-tert-butylamine) were tested in combination with
B(C6F5)3, as such FLPs have been shown to effect heterolytic
cleavage of H2.18c However, in all cases, treatment with N2O in
bromobenzene failed to generate the analogous N2O complexes.
Evidently, N2O activation is strongly dependent on the nature of
the base suggesting that both strong s-donation and p-acceptance stabilize the PN2O fragment. Phosphines that are less
sterically demanding than tBu3P either form strong adducts with
Scheme 3 FLP formation from Cy3P and B(C6F4-p-H)3 and subsequent
reaction with N2O to give 8.
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Lewis acids such as B(C6F5)3, or result in nucleophilic aromatic
substitution at the para position of a C6F5 ring, as is the case with
Cy3P.21 Previously, it has been shown that the borane B(C6F4-pH)3 is immune to attack at the para-carbon atom, forming an
FLP with Cy3P.22 The reaction of an equimolar mixture of Cy3P
and B(C6F4-p-H)3 in bromobenzene with an atmosphere of N2O
results in formation of Cy3P(N2O)B(C6F4-p-H)3 8 in 59% isolated yield (Scheme 3). The somewhat lower yield of 8 compared
to its tBu3P-analogue 6 is presumably the result of the more facile
oxidation of Cy3P by N2O. Indeed, monitoring a solution of
Cy3P in C6D5Br under an atmosphere of N2O by 31P NMR
showed >90% conversion to Cy3P]O overnight, whereas oxidation of tBu3P with N2O is more sluggish (50% conversion
overnight).23
Spectroscopic analysis of 8 revealed a single resonance in the
31
P{1H} NMR spectrum at 56.4 ppm. Two signals, representative
of the ortho and meta fluorines, were observed in the 19F NMR
spectrum at 134.3 and 143.0 ppm as compared to 130.2 and
139.3 in the free borane.22 A sharp singlet in the 11B{1H} NMR
spectrum at 0.8 ppm was observed, typical of a four-coordinate
boron center. The isotopically labeled compound 8-15N shows the
expected coupling patterns. Comparison with the 15N spectroscopic data for the tBu3P analogue 5-15N reveals no significant
difference in the 1JN–N coupling constant, whereas 1JP–N is
smaller by 7.2 Hz and 2JN–P larger by 2.1 Hz for 8-15N (Table 1).
A crystallographic study unambiguously confirmed the structure
of 8 (Fig. 4, pertinent bond lengths and angles in Table 2).
The analogous zwitterionic species using the considerably
weaker Lewis acid tris(para-fluorophenyl)borane was sought. An
equimolar mixture of B(C6H4-p-F)3 and Cy3P in bromobenzene
was allowed to stir overnight under an atmosphere of N2O.
Precipitation with pentane yielded a microcrystalline colourless
solid. NMR spectroscopy in CD2Cl2 showed singlets in the
11
B{1H}, 19F and 31P{1H} NMR spectra at 26.4, 116.4 and 62.6
ppm respectively. Subsequent recrystallization of the crude
product from a layered CH2Cl2–pentane solution at 35 C
yielded colorless crystals of Cy3P]O(B(C6H4-p-F)3), 9. A crystallographic study confirmed the identity of 9 (Fig. 5). Efforts to
observe an intermediate N2O adduct en route to 9 were unsuccessful. The results above are in agreement with the notion that
the zwitterionic compounds R3P(N2O)BR0 3 are intermediates in
Staudinger-type oxidations24 that are kinetically trapped by
coordination of the Lewis acidic borane fragment. This kinetic
stabilization is only successful when Lewis acid coordination is
sufficiently strong (irreversible) and faster than phosphine oxidation. In this sense, these compounds are analogues of the
phosphazides R3P(NNN)R0 .25
Lewis acid exchange reactions
Fig. 4 Molecular structure of 8. Hydrogen atoms have been omitted for
clarity.
Fig. 5 POV-ray depiction of the molecular structure of 9. Hydrogens
and angles ( ):
have been omitted for clarity. Selected bond distances (A)
P(1)–O(1) 1.5251(8); B(1)–O(1) 1.5854(15); P(1)–O(1)–B(1) 147.26(8).
This journal is ª The Royal Society of Chemistry 2011
Further investigation of the use of the weakly acidic borane
B(C6H4-p-F)3 for the activation of N2O in conjunction with
t
Bu3P revealed that the borane remains only weakly bound to the
t
Bu3PN2O fragment allowing for facile exchange of the acidic
fragment for a stronger Lewis acid.12 Following this method, 1
can easily be generated via the reaction of 6 with an equivalent of
B(C6F5)3. Similarly, treatment of 6 with an equivalent of trityl
tetrakis(pentafluorophenyl)borate in CD2Cl2 at room temperature immediately yielded a bright yellow solution (Scheme 4).
NMR spectroscopic analysis revealed a new product with a
single resonance in the 31P{1H} NMR at 76.3 ppm, considerably
downfield from that in 6 (65.4 ppm). NMR spectroscopy indicated the liberation of free B(C6H4-p-F)3, which was easily
removed by precipitation of the ionic product and washing with
pentane. Synthesis of the 15N isotopologue 10-15N supported that
the tBu3P(N2O) fragment remains intact throughout the
exchange process, as evidenced by two doublets of doublets in
the 15N NMR spectrum at 548.4 and 412.3 ppm, respectively,
with a N–N coupling constant of 14.7 Hz. Similarly the 31P{1H}
NMR spectrum revealed a doublet of doublets with 1JP–N and
Scheme 4 Exchange of the borane fragment B(C6H4-p-F)3 of 6 by trityl
cation generating 10.
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Fig. 6 Molecular structure of the cation of 10. Hydrogens and the
B(C6F5)4 counter ion have been omitted for clarity.
Table 3 Selected metrical parameters for 10–13a
10 (X ¼ C)
11 (X ¼ Zr)
13 (X ¼ Zr)
P(1)–N(1)
N(1)–N(2)
N(2)–O(1)
O(1)–X(1)
1.7217(13)
1.2418(17)
1.3752(15)
1.4955(17)
1.719(4)
1.233(5)
1.372(5)
2.105(3)
1.683(3)
1.266(4)
1.300(4)
2.151(3)
P(1)–N(1)–N(2)
N(1)–N(2)O(1)
N(2)–O(1)X(1)
115.98(10)
108.52(12)
110.68(10)
111.3(3)
111.0(3)
126.2(2)
116.9(3)
111.9(3)
123.4(2)
a
angles in degrees ( ).
Bond lengths in angstroms (A),
2
JP–N of 58.2 and 20.5 Hz. These data are similar to those
observed for 1-15N to 7-15N although there is a marked shift for
the 15N NMR signals (Table 1). Crystals of 10 were grown from a
layered CH2Cl2/cyclohexane solution at room temperature and
an X-ray crystallographic study unambiguously confirmed its
solid state structure as [tBu3P(N2O)CPh3][B(C6F5)4] (Fig. 6,
pertinent bond distances and angles in Table 3).
Throughout the exchange process the tBu3P(N2O) fragment
remains intact and appears to be inherently stable. It is worth
noting that this method of borane exchange also offers an easy
and clean route to the generation of 2, 3, 4, 5 and 7 via exchange
of 6 with one equivalent of PhB(C6F5)2, MesB(C6F5)2,
(C6F5)2BOC6F5, B(C6F4-p-H)3 or with half an equivalent of
1,4-(C6F5)2B(C6F4)B(C6F5)2.
Extending this borane exchange strategy to early transition
metal complexes is useful as there are few documented cases
where intact N2O interacts with metal complexes without the
Scheme 5 Synthetic scheme for the generation of 11 and 12 by Lewis
acid exchange.
174 | Chem. Sci., 2011, 2, 170–176
Fig. 7 The molecular structure of the cation of 11. Hydrogens and the
counter anion MeB(C6F5)3 have been omitted for clarity.
decomposition of N2O to O and N2.10 Thus, treatment of a
bromobenzene solution of [Cp2ZrMe][MeB(C6F5)3] with an
equivalent of 6 resulted in a beige oil that solidified upon trituration with pentane. Recrystallization from CH2Cl2–pentane at
35 C afforded 11 in 83% isolated yield (Scheme 5). NMR
spectroscopic studies revealed a singlet in the 31P{1H} NMR at
67.7 ppm. Three peaks were observed in the 19F NMR spectrum
pertaining to the ortho, para and meta signals of the MeB(C6F5)3
anion at 133.1, 165.3 and 167.8 ppm, with no traces
of residual B(C6H4-p-F)3. A broad signal in the 1H NMR at
0.48 ppm was observed pertaining to the methyl group of the
counter anion, MeB(C6F5)3. With these spectroscopic data,
[tBu3P(N2O)ZrCp2Me][MeB(C6F5)3] was proposed as the structure of 11, which was confirmed by a single crystal X-ray diffraction study (Fig. 7, Table 3).
The crystallographic characterization of 11 constitutes the
only example of an intact N2O fragment where the oxygen of the
N2O fragment is bound directly to a zirconium center.10 As
observed in the tBu3P(N2O)BR3 compounds described above, the
molecular structure displays the same trans orientation of the
ZrO and tBu3P fragments relative to the N–N bond. The metric
parameters for the PN2O fragment in 11 are similar to those
observed in the borane compounds described above. The N(2)–
O(1)–Zr(1) angle of 126.2(2) is relatively large in comparison to
the corresponding N–O–B or N–O–C angles in 1–10 (110.32(13)–
114.86(14) ), which may reflect some degree of interaction
between an O lone pair and a vacant metal-based orbital that is
absent in the main group species 1–10. Coordination of the
t
Bu3P(N2O) fragment to the cationic zirconocene center results
in little structural changes in the Cp2ZrMe+ moiety. The
Cp(centroid)–Zr–Cp(centroid) angle in 11 is 130.5 , which may
be compared to 129.6 in [Cp2ZrMe(THF)][BPh4]26 or 129.2 in
and Zr–O (2.105(3)
Cp2ZrCl2.27 As well, the Zr–Me (2.261(4) A)
A) bond lengths are statistically indistinguishable from the corresponding distances in Cp2ZrMe(THF)+ (2.256(10) and
respectively).26 In a similar manner, Lewis acid
2.122(14) A,
exchange between [Cp2TiMe][MeB(C6F5)3] and 6 resulted in the
formation of [tBu3P(N2O)TiCp2Me][MeB(C6F5)3] 12, in 81%
yield. These species were found to be stable in solution for a
number of days.
It is noteworthy that attempts to generate 11 directly from the
reaction of the FLP generated by equimolar mixture of
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Scheme 6 Activation of N2O employing a Zr/P FLP yielding 13. Cp* ¼
C5Me5.
[Cp2ZrMe][MeB(C6F5)3] and with 15N2O gave only inseparable
and unidentified reaction products. However, in contrast, an
analogous species, generated by the zirconocene methoxide cation [Cp*2Zr(OMe)][B(C6F5)4] and an equivalent of tBu3P in
bromobenzene (Scheme 6) reacted under an atmosphere of N2O
for 12 h, to give a yellow solid, 13, which was isolated in 74%
yield (Scheme 6). Spectroscopic analysis revealed a single resonance in the 31P{1H} NMR at 64.4 ppm while the 1H NMR data
showed resonances attributable to the Cp* and methoxide
ligands at 1.91 and 3.98 ppm, respectively. The 11B{1H} and 19F
NMR spectra were in keeping with the corresponding B(C6F5)4
counter anion. In addition, 15N and 31P NMR spectroscopy on
the isotopically labeled compound 13-15N is indicative of an
intact tBu3P(N2O) fragment (Table 1). The sum of these data
firmly supports the formulation of 13 as [tBu3P(N2O)ZrCp*2(OMe)][B(C6F5)4].
An X-ray structure determination of 13 (Fig. 8, pertinent bond
lengths and angles in Table 3) confirmed that the N2O fragment
is O-bound to the Zr center and N-bound to the P center of the
phosphine while the orientation remains trans about the N–N
bond. Based on the observation of quite disparate Zr–O bond
distances, the OMe and ON2tBu3P moieties interact with the
metal center in a rather different manner. The Zr–O(alkoxide)
is typical for zirconocene alkbond length in 13 of 1.916(2) A
t
28
oxides (e.g., [Cp2Zr(O Bu)(THF)][BPh4]: average 1.890(4) A)
t
but significantly shorter than the Zr–ON2 Bu3P distance of
In addition, the large Zr(1)–O(2)–C(21) angle of
2.151(3) A.
169.9(2) in comparison to Zr(1)–O(1)–N(2) (123.4(2) ) suggests
that the methoxide ligand, but not the ON2tBu3P fragment, is
engaged in substantial backdonation to the electron deficient
metal center.28,29 A comparison between 11 and 13 shows that the
Zr(1)–O(1) bond is shortest in 11, in agreement with it having the
most Lewis acidic metal center. The P(1)–N(1) and N(2)–O(1)
respectively, were
bond lengths in 13 of 1.683(3) and 1.300(4) A,
found to be shorter than those in 11 (1.719(4) and 1.372(5) A).
Conversely, the N(1)–N(2) bond in 13 is somewhat longer
in 11). Unlike the P/B systems where
(1.266(4) vs. 1.233(5) A
variation of the Lewis acidic fragment has been shown to have
very little effect on the nature of the N2O moiety, variation in the
Lewis acidity of the metallocene cation causes a more significant
perturbation of the PN2O fragment.
Mechanism of borane exchange
In order to obtain mechanistic insight, a kinetic study of Lewis
acid exchange reactions was undertaken. Thus, to a solution of
t
Bu3P(N2O)B(C6H4-p-F)3 6 in CD2Cl2 at room temperature was
added an equimolar amount of a stronger Lewis acid (e.g.,
B(C6F5)3, B(C6F5)2Ph). The NMR spectra taken immediately
after mixing (<5 min) in all cases showed only the presence
of exchange products, suggesting that these reactions are fast.
It is likely that the weakly Lewis acidic nature of the borane
in 6 results in facile exchange. Employing a species containing a more tightly bound Lewis acid, the trityl compound
[tBu3P(N2O)CPh3][B(C6F5)4] 10 was shown to react cleanly
with B(C6F5)3 to give [Ph3C][B(C6F5)4] and tBu3P(N2O)B(C6F5)3
(1). However, even when CD2Cl2 was condensed at 196 C into
an NMR tube containing solid 10 and B(C6F5)3, NMR spectroscopic analysis of the reaction at 80 C showed full conversion to the expected products before the first spectrum could
be measured. It was thus not possible to obtain rate data. As an
alternative, the exchange between tBu3P(N2O)B(C6F5)3 (1) and
free B(C6F5)3 was examined by 19F NMR spectroscopy. Wellseparated, sharp resonances for both species are observed in
the 19F NMR spectrum, but 2D 19F EXSY experiments30 (smix ¼
1.2 s) showed no evidence of exchange between the two species
up to 80 C. Gratifyingly, exchange was observed between
t
Bu3P(N2O)B(C6H4-p-F)3 (6) and free B(C6H4-p-F)3 at room
temperature and analysis of 2D 19F EXSY spectra obtained
between 25 and +25 C allowed determination of the activation parameters of the exchange process as DH‡ ¼ 71.2(9)
kJ mol1 and DS‡ ¼ 32(3) J mol1 K1 (see Supporting Information for details). The small, positive value for DS‡ suggests
that the B–O linkage is somewhat weakened before the
incoming borane binds. The lack of observable exchange (i.e., a
significantly higher activation barrier) between 1, in which the
borane is more strongly bound, and B(C6F5)3 is in agreement
with this proposal.31
Conclusions
Fig. 8 The molecular structure of 13. Hydrogens and the counter anion
B(C6F5)4 have been omitted for clarity.
This journal is ª The Royal Society of Chemistry 2011
FLPs react with N2O under mild conditions to give zwitterionic
products R3P(N2O)BR2R0 in high yield. Phosphines with
diminished steric demands or Lewis basicity, or nitrogen Lewis
bases do not give analogous compounds, suggesting that a
combination of strong s-donation and p-acceptance is required
to capture the N2O fragment. A range of borane Lewis acidities
have been studied. These show little difference in the geometric
Chem. Sci., 2011, 2, 170–176 | 175
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or electronic properties of the resulting N2O moieties. The
robustness of the R3P(N2O) moiety allows the synthesis of otherwise inaccessible N2O species by exchange of the borane for
stronger Lewis acids such as Ph3C+ and [Cp2ZrMe]+. This route
to N2O transition metal derivatives offers a new strategy to
exploring and understanding the nature of N2O binding, transport and reduction in synthetic as well as biological systems.
Further studies of both main group and transition N2O species
are underway and will be reported in due course.
Downloaded on 09 September 2011
Published on 13 September 2010 on http://pubs.rsc.org | doi:10.1039/C0SC00398K
Acknowledgements
DWS gratefully acknowledges the financial support of NSERC
of Canada and the award of a Canada Research Chair and
a Killam Research Fellowship. EO is grateful for the support of a
Rubicon postdoctoral fellowship from the Netherlands Organisation for Scientific Research (NWO). RCN is grateful for the
award of an Ontario Graduate Scholarship.
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