A mechanistic study of the C-P bond cleavage reaction of 1,2-(PH 2)2-C6H4 with nBuLi/ Sb(NMe2)3

Article abstract of DOI:10.1039/b810028d

In situ ³¹P NMR spectroscopic studies of the reaction of the primary diphosphine 1,2-(PH₂)₂-C₆H₄ with the mixed-metal base system ⁿBuLi/Sb(NMe₂) ₃, combined with X-ray structural investigations, strongly support a mechanism involving a series of deprotonation steps followed by antimony-mediated reductive C-P bond cleavage. The central intermediate in this reaction is the tetraphosphide dianion [C₆H₄P ₂]₂^2- ([4]) from which the final products, the 1,2,3-triphospholide anion [C₆H₄P₃]^- (3) and [PhPHLi] (8·Li), are evolved. An EPR spectrocopic study suggests that homolytic C-P bond cleavage is likely to be involved in this final step.

Full text of DOI:10.1039/b810028d

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A mechanistic study of the C–P bond cleavage reaction of 1,2-(PH2)2-C6H4
with nBuLi/Sb(NMe2)3†
Ruth Edge,b Robert J. Less,*a Vesal Naseri,a Eric J. L. McInnes,b Robert E. Mulveyc and Dominic S. Wright*a
Received 13th June 2008, Accepted 7th August 2008
First published as an Advance Article on the web 8th October 2008
DOI: 10.1039/b810028d
In situ 31P NMR spectroscopic studies of the reaction of the primary diphosphine 1,2-(PH2)2-C6H4 with
the mixed-metal base system nBuLi/Sb(NMe2)3, combined with X-ray structural investigations,
strongly support a mechanism involving a series of deprotonation steps followed by antimony-mediated
reductive C–P bond cleavage. The central intermediate in this reaction is the tetraphosphide dianion
[C6H4P2]2 ([4]) from which the final products, the 1,2,3-triphospholide anion [C6H4P3]- (3) and
2-
[PhPHLi] (8·Li), are evolved. An EPR spectrocopic study suggests that homolytic C–P bond cleavage is
likely to be involved in this final step.
Introduction
In recent studies, we have begun to see parallels between the
reactions of main group complexes1 with primary phosphines
[RPH2] and those involving transition metals.2 In particular,
Scheme 2
Stephan and co-workers showed that the reactions of [Cp*ZrCl2]
with primary aliphatic phosphide anions [RPH]- result in P–P
bond formation, producing [Cp*2Zr(PR)n] via a dehydrocoupling
of the organic precursor (Scheme 3, top).6 The ultimate fate of
the antimony component is the Zintl anion Sb11 (at least in the
3-
=
mechanism involving the transient phosphinidenes [Cp*2Zr PR]
case of the reaction undertaken in the presence of 12-crown-4).5
Although full mechanistic details of this unique reaction were not
available at the time, some overall features are worthwhile noting.
Firstly, the reaction appears to be driven thermodynamically by
the large value of the P–P bond energy,7 which is the highest
homoatomic single-bond bond energy between any of the group 15
elements. Secondly, we showed that the low-temperature reaction
of 1,2-(PH2)2C6H4 with nBuLi/Sb(NMe2)3, in the presence of the
Lewis base donor ligand TMEDA (Me2NCH2CH2NMe2), results
in the formation of the formally 6p aromatic anion [C6H4P2Sb]-
(1), which couples spontaneously into the distibane tetraanion
(Scheme 1).2a,c We find a similar formation of oligophosphanes
occurs in the reactions of [RPH]- with mixed-metal base systems of
the type RM/E(NMe2)3 (M = alkali metal; E = As, Sb, Bi), giving
intermediate heterocycles of the type [(RP)nE]-.3 The latter anions
ultimately eliminate cyclic phosphanes [RP]n, with the formation
of Zintl ions like [Sb7]3-.1,3,4 An even closer link is seen in the
reaction of PhCH2Na/Sn(NMe2)2 with [MesPH]- (Mes = 2,4,6-
Me3C6H4), which,5 like the reaction with [Cp*2ZrCl2],2a also leads
to C–H bond activation of the ortho-CH3 groups of the Mes ligand,
resulting in the formation of the stannate trianion [Sn(1-P-2-CH2-
C6H4)(PMes)]3- (Scheme 2).
[C6H4P2Sb]2 (2).8 These species are potential intermediates in
4-
the initial stage of the formation of the 1,2,3-triphospholide
anion 3, through which the observed oxidation of the phosphorus
component and reduction of the antimony component might be
mediated. In contrast to the main group system, the reaction
of the transition metal anion [Cp*2ZrH3]- with 1,2-(PH2)2C6H4
follows a seemingly different course, producing the cyclic octamer
[C6H4P2]8 (5) via the dimeric organophosphane [C6H4PPH]2 (4H2)
(Scheme 3, bottom).2a We show in this paper, however, that both
of these reactions are closely related mechanistically; involving the
same type of dimeric organophosphane intermediate but with the
key differences that, (i) the ultimate formation of a macrocycle
(like 5) is intercepted by C–P bond cleavage in the main group
system, and (ii) that unlike the transition metal reaction, the main
group reaction is stoichiometric rather than catalytic.
Scheme 1
Remarkably, we recently observed that the reactions of 1,2-
(PH2)2-C6H4 with the mixed-metal base nBuLi/Sb(NMe2)3 at
reflux in THF (in the presence or absence of various Lewis base
donors) to give the 1,2,3-triphospholide anion [C6H4P3]- (3), in
which a P atom has been inserted into the original framework
aChemistry Department, Cambridge University, Lensfield Road, Cambridge,
UK CB2 1EW. E-mail: dsw1000@cam.ac.uk.
bSchool of Chemistry, The University of Manchester, Oxford Road, Manch-
ester, UK M139PL
In situ 31P NMR and EPR spectroscopic studies
cThe Department of Pure and Applied Chemistry, University of Strathclyde,
Glasgow, Scotland, UK G1 1XL
Our first task in the elucidation of the mechanism of the reaction
producing the 1,2,3-triphospholide anion 3 was to monitor it
† CCDC reference numbers 691462 and 691463. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b810028d
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Scheme 3
using in situ 31P NMR spectroscopy. Although our original, largely
synthetic study, was undertaken in THF–12-crown-4 (producing
the crystalline complex [C6H4P3]-[Li(12-crown-4)2]+),6 we had
noted that in situ 31P NMR spectra of the final reaction solutions
of 1,2-(PH2)2-C6H4 with Sb(NMe2)3 after reflux showed that 3
is produced as the major product in a range of solvents and
in the presence of various Lewis base donors including THF
alone and TMEDA–THF. In situ 31P NMR spectroscopic studies
undertaken as part of the current work at room temperature
not only confirm this but also reveal that the same phosphorus-
containing intermediates are involved in the presence of different
donors and solvents. Fig. 1 shows key spectra for the 1 : 2 : 0.33 stoi-
chiometric reaction of 1,2-(PH2)2-C6H4 with nBuLi and Sb(NMe2)3
undertaken with THF–TMEDA (3 : 1 volumetric ratio) as the
solvent and monitored as a function of time and temperature.
The results in THF–TMEDA are not only representative of all of
the solvent–donor mixtures investigated, but produced the most
well-resolved 31P NMR spectra.
and the dilithiate [1,2-(PH)2-C6H4]2- (6) (s, d = -123.7 ppm). As
expected, in the fully-coupled 31P spectrum, the resonance for 1
remains a singlet, whereas that for the dilithiate precursor 6 reverts
to an apparent double-doublet, which is actually the AA¢ part of an
AA¢XX¢ system.9 The coupling constants obtained from algebraic
analysis of the spectrum [|JXX¢| = 262.3, |JAX| = 172.0, |JAX¢| =
50.7 and |JAA¢| = 9.2 Hz] are similar to those found previously for
the dilithiates [1,2-(RP)2-C6H4]Li2 (R= Me3Si, Ph).10 In addition,
the assignment of this species to 6 is proved by the later structural
characterisation of the complex 6·(Li·TMEDA)2.
1
The 31P{ H} NMR spectra of the solution, obtained after
allowing the reaction to warm to room temperature (Fig. 1b) and
stirring for 1 h (Fig. 1c), show that while some of the dilithiate 6
remains, all of anion 1 has been consumed. In addition, three
new major species are observed. The singlet resonance found
at d = -85.5 ppm is almost identical to that observed in an
4-
authentic sample of the tetraanion [C6H4P2Sb]2 (2), in the form
of the previously reported complex 2·[Li·TMEDA]4 (in d8-THF).8
2-
Another species that is easily identified is the dianion [C6H4P2]2
([4]), which is observed as both parts of an AA¢XX¢ systems at
d = +6.8 ppm (P2,3) and -51.1 ppm (P1,4) [|JAX| = 346.6, |JAX¢| =
1.6 Hz, |JXX¢| 253.9, |JAA¢| = 5.6 Hz].9 It can be noted that
these resonances differ markedly from those found for neutral
[C6H4PPH]2 (4H2) [d = -4.2 ppm (P2,3) and -48.6 ppm (P1,4)]2c and
that unlike this neutral diphosphine, the two AA¢XX¢ multiplets
for the dianion [4] are almost unaltered in the fully-coupled 31P
NMR spectrum of the reaction solution. In addition, the presence
of the monoanion [(C6H4)2PP(H)]- (d = -19.4 and -65.1 ppm)2b
and the octameric phosphorus macrocycle [C6H4P2]8 (5) (AA¢XX¢
spin system, d = -22.5 and -65.4 ppm)2a can also be excluded. Fi-
nal support for our assignment was provided by the later structural
characterisation of [C6H4P2]2(Li·PMDETA)2 [4·(Li·PMDETA)2]
[PMDETA= (Me2CH2CH2)2NMe]. The remaining intermediate
can be assigned tentatively to the [C6H4P3C6H4PH] (7) anion,
resulting from protonation of the dianion [4] with concomitant
P–P bond cleavage (Scheme 4). Although we have been unable
to simulate the 31P NMR spectrum of this species, the resonance
for PA and PB (the A2B part of this A2BX spin system) appears
as an apparent eight line, (presumably) second-order multiplet
1
Fig. 1 In situ 31P{ H} NMR studies of the reaction of Sb(NMe2)3,
nBuLi and 1,2-(PH2)2-C6H4; (a) taken immediately after mixing of the
components at -78 ◦C, (b) after heating to room temperature, (c) after
stirring for an additional 1 h at 25 ◦C, (d) after heating to 116 ◦C for 2 h
in neat TMEDA.
1
at d = ca. -50 ppm in the 31P{ H} NMR spectrum. More
The 31P{ H} NMR spectrum of the mixture of Sb(NMe2)3 with
1
diagnostic, however, is the appearance of the P(H) phosphorus
atom (the PX part of the spin system) at a distinct chemical shift of
d = -134.3 ppm as a doublet of triplets, from which the apparent
coupling constants JAX = 37.2 and JBX = 145.8 Hz can be
1,2-(PH2)2-C6H4 in THF–TMEDA obtained immediately after the
initial reaction at -78 ◦C (Fig. 1a) shows the presence of two major
species, the 6p-monoanion [C6H4P2Sb]- (1) (s, d = 309.6 ppm)
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Scheme 4 Formation of the proposed intermediate 7 from [4]2-.
extracted. Further support for the assignment of this intermediate
is provided by the fully-coupled 31P NMR spectrum of the reaction
solution in which the PX resonance splits into a doublet of doublet
of triplets (1JPH = 173.0 Hz), with additional minor splitting of
the A2B part of the spectrum also being observed. Significantly, a
1
closer inspection of the 31P{ H} NMR spectrum at this stage also
identifies the presence of [PhPH]- (8) as a minor singlet resonance
at d = -112.7 ppm (1JPH = 170 Hz) (literature d -112.6 ppm,
1JPH = 170 Hz),11 which splits into the expected doublet in the
fully-coupled spectrum.
Heating of the reaction mixture to 116 ◦C for 2 h in neat
TMEDA results in drastic or complete depletion of all of the
intermediates 2, [4] and 7, as observed in Fig. 1d. The only
major species remaining are the dilithiate precursor [1,2-(PH)2-
C6H4PH]Li2 (6·Li2), [PhPHLi] (8·Li) and the product 1,2,3-
triphospholide anion [C6H4P3]- (3) (an A2B system previously
observed for this anion). The appearance of this final spectrum
can be compared to our previous preliminary report of the in situ
spectrum of the final reaction solution of Sb(NMe2)3 with 1,2-
(PH2)2-C6H4 in THF–12-crown-4, which showed almost exclusive
formation of the complex [C6H4P3][Li(12-crown-4)2] (3·[Li(12-
crown-4)2]), with only trace byproducts being observed after reflux
for 1 h in THF.8
Fig. 3 Structure of dilithiate molecules of 6·(TMEDA)2. H atoms have
◦
˚
been omitted for clarity. Key bond lengths (A) and angles ( ); P(1)–Li(2)
2.555(4), P(1)–Li(1) 2.564(5), P(2)–Li(1) 2.567(5), P(2)–Li(2) 2.603(5),
N–Li range 2.068(6)–2.107(6), Li–P–Li range 103.9(2)–105.4(2), P–Li–P
range 74.0(1)–73.6(1), butterfly angle between the LiP2 planes 162.5.
[C6H4P3]∑ or the 7p species [C6H4P3]2-∑, valence-isoelectronic with
∑
the [C6H4P2Sb]22- (2)8 and benzothiazolyl [C6H4S2N] radicals.13
Supporting synthetic and structural studies
In order to provide further support for our assignments of the
intermediates gleaned from the above 31P NMR spectroscopic
studies, we attempted to isolate and fully characterise some of
the intermediates of the reaction. Lithiation of the diphosphine
1,2-(PH)2-C6H4 with nBuLi in the presence of TMEDA (1 : 2 : 2)
in THF at -78 ◦C gives a bright red-orange solution from
which the crystalline complex [1,2-(PH)2-C6H4](Li·TMEDA)2
[6·(Li·TMEDA)2] was isolated in 73% yield (Scheme 5). A brief
report of this complex has appeared previously, although no solid-
state structure had been obtained.10 For completeness, it was
characterised by 1H, 31P and 7Li NMR spectroscopy prior to
obtaining its low-temperature X-ray structure (see Experimental).
It can be noted that, contrary to the previous report, the 31P
A last intriguing feature of the spectroscopic study of this
reaction system is the observation of an abundant radical in
the reaction mixture at 67 ◦C by EPR (electron paramagnetic
resonance) spectroscopy (Fig. 2). The X-band EPR spectrum is
shown in Fig. 3. The appearance of a doublet of triplets in this
spectrum (g = 2.0119, a = 29 G, a = 20 G) strongly suggests
a phosphorus-based radical in which an electron is delocalised
over a P ···P···P fragment is present. The relatively large g
values seen for this radical are typical of other phosphorus-based
radicals.12 A potential candidate for this radical is the neutral 6p
10
NMR of crystalline 6·(Li·TMEDA)2 is actually well resolved.
This spectrum is nearly identical to that assigned to the anion 6 in
the earlier in situ NMR spectroscopic studies, confirming (perhaps
unsurprisingly) that the [1,2-(PH)2-C6H4]2- anion is indeed present
as a precursor in the reaction and that it can be generated in the
absence of other (mono or tri) lithiates of the phosphine. Heating
n
this reaction mixture to reflux for 1 h, in the presence of BuLi,
Sb(NMe2)3 gives the expected product [C6H4P3][Li(TMEDA)2]
[3·(Li·TMEDA)2] in 26% yield, which was also fully characterised
(Scheme 5) (see Experimental). In this case, however, only poor
quality X-ray data could be obtained despite repeated attempts.
The thermal instability of the anion [C6H4P2Sb]- (1) is in marked
contrast to the arsenic analogue [C6H4P2As]- (9), which is readily
obtained as the new complex 9·(Li·TMEDA)2 by the reaction
of 6·(Li·TMEDA)2 with As(NMe2)3 in 47% yield (Scheme 5)
(see Experimental). The synthesis of the same anion has only
previously been reported in low yield.14 It is worthwhile noting
Fig. 2 X-Band EPR spectrum of the reaction of Sb(NMe2)3/nBuLi with
1,2-(PH2)2-C6H4 in toluene at 340 K.
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Scheme 5
that the 31P chemical shifts of the a-P atoms within the series of
anions [C6H4P2E]- correlates inversely with the electronegativity of
the heteroatom (E) [E = P, d = 260.7 ppm;6 E = As, d = 300.2 ppm;
E = Sb, d = 311.0 ppm15], consistent with the developing ionicity
of the E–P bonds as group 15 is descended.
The low-temperature [180(2) K] X-ray structure of
6·(Li·TMEDA)2 is shown in Fig. 3, with key bond lengths and
angles being included in the caption. The structure is very similar
to those reported earlier for [1,2-(PR)2-C6H4](Li·TMEDA)2]
(R= Ph, SiMe3),10 in which the 1,2-PH groups are bridged
by two TMEDA-solvated Li+ cations. The P–Li bonds in
16
˚
6·(Li·TMEDA)2 [range 2.555(4)–2.603(5) A ] and the P–Li–P
[range 74.0(1)–73.6(1)◦] and P–Li–P [range 103.9(2)–105.4(2)◦]
angles are also similar to those observed in the close-related
[1,2-(PR)2-C6H4](Li·TMEDA)2] complexes.10
Fig. 4 Structure of the monomeric dilithiate 4·(Li·PMDETA)2. H
Since no other products could be isolated using bidentate
TMEDA as the Lewis base donor under various conditions,
it was decided to change to the closely related tridentate
donor PMDETA [(Me2NCH2CH2)2NMe] in an attempt to in-
tercept the final C–P bond cleavage step. On one occasion, we
were fortunate in isolating a few crystals of the new complex
[C6H4P2]2(Li·PMDETA)2 [4·(Li·PMDETA)2] from the reaction of
nBuLi/Sb(NMe2)3 with 1,2-(PH2)2-C6H4 (2 : 0.33 : 1 equivalents)
in THF–PMDETA (see Experimental), after prolonged storage at
˚
atoms have been omitted for clarity. Selected bond lengths (A)
and angles (◦): P(1)–P(2) 2.172(1), P(2)–P(3) 2.194(1), P(3)–P(4)
2.176(1), C–P range 1.825(3)–1.846(3), P(1)–Li(1) 2.501(4), P(4)–Li(2)
2.536(4), P(1)–P(2)–P(3) 97.14(4), P(4)–P(3)–P(2) 96.62(4), C(7)–P(2)–P(1)
104.55(9), C(2)–P(3)–P(4) 106.26(8), C(2)–P(3)–P(2) 97.18(9), C(8)–P(4)–
P(3) 97.57(9), fold angle between C2P3 mean planes is 77.6.
Conclusions
◦
-15 C. Owing to the low yield of 4·(Li·PMDETA)2, it was only
From the combined NMR, EPR spectroscopic studies and
structural investigations described previously, we are now in a
position to propose at least an outline of the mechanism by
which 1,2-(PH2)-C6H4 is converted into the 1,2,3-triphospholide
anion [C6H4P3]- (3) (Scheme 6). Our studies confirm that this
reaction occurs by initial stepwise deprotonation of 1,2-(PH2)2-
C6H4 by nBuLi, generating the [1,2-(PH)2-C6H4]2- dianion, which
is then converted into the [C6H4P2Sb]- anion by further depro-
tonation with Sb(NMe2)3. This anion is rapidly converted into
the tetraanion 2, which is short-lived at room temperature in
solution. It is not clear from in situ 31P NMR spectroscopic
characterised by X-ray diffraction. The low-temperature [180(2)
K] X-ray structure of 4·(Li·PMDETA)2 (Fig. 4) shows that it is
a monomeric, ion-paired complex in the solid state. There is a
2-
noticeably more acute folding of the [C6H4P2]2 dianion about
the P(2)–P(3) bond vector (77.6◦ between the mean planes of the
C2P3 rings) compared to the parent diphosphine [C6H4PPH]2 [4H2]
(88.6◦).2c Surprisingly, considering the increased potential for
lone-pair–lone-pair repulsion, there is actually a small reduction
in all of the P–P bond lengths in the [C6H4P2]2- dianions of
15
˚
4·(Li·PMDETA)2 [range 2.172(1)–2.194(1) A] compared to 4H2
2c
2-
˚
[range 2.1952(7)–2.2085(7) A].
studies whether the [C6H4P2]2 dianion ([4]) or the supposed
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