Syntheses and redox properties of the first phosphirene-dinitrogen and phosphirene-diazenide complexes

Article abstract of DOI:10.1039/a906238f

The first mixed phosphirene-dinitrogen and phosphirene-diazenide complexes mer-[ReCl(N₂)(PPhCPh=CPh)L₃] (L = PMe₂Ph 1a or PMePh₂ 1b) and [ReBr(NNPh)₂(PPhCPh=CPh)₂(PPh₃)] 2 have been prepared by treatment of the corresponding thf solutions of trans-[ReCl(N₂)L₄] or [ReBr₃(NNPh)(PPh₃)₂] with PPhCPh=CPh. Their redox properties have been investigated by cyclic voltammetry in an aprotic medium, at a platinum electrode, and the electrochemical E_L and P_L parameters estimated for the phosphirene ligand indicating that its overall electron donor/acceptor properties are similar to those of PMePh₂. The Royal Society of Chemistry 1999.

Full text of DOI:10.1039/a906238f

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Syntheses and redox properties of the first phosphirene–dinitrogen
and phosphirene–diazenide complexes
Armando J. L. Pombeiro,*a Maria Teresa A. R. S. Costa,a Yu Wanga and John F. Nixonb
a Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Av. Rovisco Pais,
1049-001 Lisboa, Portugal. E-mail: pombeiro@alfa.ist.utl.pt
b School of Chemistry, Physics and Environmental Science, University of Sussex, Brighton,
UK BN1 9QJ. E-mail: J.Nixon@sussex.ac.uk
Received 2nd August 1999, Accepted 7th September 1999
The first mixed phosphirene–dinitrogen and phosphirene–diazenide complexes mer-[ReCl(N )(PPhCPh᎐CPh)L ]
᎐
2
3
(L = PMe Ph 1a or PMePh 1b) and [ReBr(NNPh) (PPhCPh᎐CPh) (PPh )] 2 have been prepared by treatment of
᎐
2
2
2
2
3
the corresponding thf solutions of trans-[ReCl(N )L ] or [ReBr (NNPh)(PPh ) ] with PPhCPh᎐CPh. Their redox
᎐
2
4
3
3 2
properties have been investigated by cyclic voltammetry in an aprotic medium, at a platinum electrode, and the
electrochemical EL and PL parameters estimated for the phosphirene ligand indicating that its overall electron
donor/acceptor properties are similar to those of PMePh2.
ligand by estimating, for the first time, its electrochemical EL
and PL ligand parameters.
Introduction
The chemistry of phosphirene and phosphirane ring systems
has been reviewed1,2 and also more recently discussed with
other phosphorus containing heterocycles.3 The phosphirene
ring is characterised by a significant positive charge at phos-
phorus and two bent intracyclic P–C bonds corresponding to
the bonding combination of π (phosphinidene) and π* alkyne
orbitals. A transition metal–ligand fragment can either co-
ordinate to the phosphorus lone-pair electrons or insert into
one of the P–C bonds via a transient η2-(P–C) complex. Several
complexes have been synthesised directly in the co-ordination
sphere of metals or made from the preformed heterocycle.4–7
Interestingly palladium catalysed alkyne insertions of phos-
phirenes ligated to [W(CO)5]8,9 are known and CO can also
be incorporated into the ring of phosphirene complexes10 at
high temperature. These reactions no doubt involve transient
four-membered metallacycles and in support of this several 14-
electron fragments of the type ML2 (M = Ni or Pt) readily react
with the P–C bonds of phosphirenes and phosphiranes,11–14 and
subsequently with CO under mild conditions.
In spite of this rich co-ordination chemistry, the electron
σ-donor and π-acceptor characters of the phosphirenes have
not yet been investigated in detail. In addition, mixed com-
plexes of phosphirenes with dinitrogen (N2) or potentially
derived ligands, such as diazenides (NNR), have not been
reported to date although organophosphine complexes with
such nitrogen ligands are widely known15,16 and present a
versatile chemistry which is of current and growing interest.
In attempting to address these points, we have investigated
Results and discussion
Chemical studies
Treatment of a thf solution of trans-[ReCl(N2)L4] (L = PMe2Ph
or PMePh ) with PPhCPh᎐CPh, in a stoichiometric amount,
᎐
2
for ca. 2–3 d, leads to the formation of the corresponding
mixed dinitrogen–phosphirene complexes mer-[ReCl(N2)-
(PPhCPh᎐CPh)L ] (L = PMe2Ph 1a or PMePh2 1b) via
᎐
3
replacement of one of the phosphine ligands by the phos-
phirene, eqn. (1). Although N2 is often the most labile ligand
trans-[ReCl(N )L ] ϩ PPhCPh᎐CPh →
᎐
2
4
mer-[ReCl(N )(PPhCPh᎐CPh)L ] ϩ L (1)
᎐
2
3
in transition metal dinitrogen complexes, in this reaction it
is retained as was observed previously by us17a in the formation
of mer-[ReCl(N2)(CNMe)(PMe2Ph)3] on treatment of trans-
[ReCl(N2)(PMe2Ph)4] with CNMe.
Complexes 1a and 1b were isolated (ca. 40–50% yields) as a
pale orange or a dark yellow solid, respectively, having strong
IR bands readily assigned to ν(N᎐N) at 1944–1930 cmϪ1, lying
᎐
᎐
within the range of frequencies (1950–1920 cmϪ1) displayed
by other N2 complexes of related electron-rich rhenium
centres, typified by mer-[ReCl(N2)(CNMe)(PMe2Ph)3]17a or
mer-[Re(S2PPh2)(N2)(PMe2Ph)3],20 in which N2 is trans to a
strong electron-donor anionic co-ligand and behaves as an
effective π-electron acceptor. This is also a feature of our
complexes 1a and 1b (see below) which accounts for the
the reactivity of triphenylphosphirene PPhCPh᎐CPh, with the
᎐
dinitrogen and the phenyldiazenide complexes trans-[ReCl-
(N2)L4] (L = PMe2Ph or PMePh2) and [ReBr3(NNPh)(PPh3)2],
respectively, since these types of complexes are particularly
promising synthetic starting materials in co-ordination
chemistry of nitrogen ligands.15–19 We have obtained from
these reactions the first dinitrogen and diazenide complexes
with a phosphirene co-ligand, mer-[ReCl(N )(PPhCPh᎐CPh)-
᎐
2
L3] (L = PMe2Ph 1a or PMePh2 1b) and [ReBr(NNPh)2-
(PPhCPh᎐CPh) (PPh )] 2, respectively and have investigated
᎐
2
3
their redox behaviour which allowed us to quantify the net
electron donor/acceptor ability of the triphenylphosphirene
J. Chem. Soc., Dalton Trans., 1999, 3755–3758
This journal is © The Royal Society of Chemistry 1999
3755

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stabilization of their Re–N2 bond which is preserved during the
reaction.
(one-electron donor) whereas the other one should be singly
bent (three-electron donor) in order to provide the complex
with the inert gas electron configuration. Such an arrangement
of phenyldiazenide conformations was shown19 by an X-ray
study on [ReBr2(NNPh)2(PPh3)2], the other isolated product
(see above) of the attempted reaction of the parent diazenide
with the phosphirene.
The meridional arrangement of the phosphines (PB and PC)
in complex 1, with the phosphirene ligand (PA) trans to one of
them (PB) (see I), implies that the N2 and the Cl ligands are
mutually trans and is clearly indicated by the [ABC2] patterns
exhibited by their 31P-{1H} NMR spectra.
The phosphirene-31P resonance (PA at δ Ϫ119.71 1a or
Ϫ131.10 1b relative to H3PO4) occurs at a lower field than that
observed for the “free” ligand (δ Ϫ186.37) and lies within the
expected range5,6 for terminal phosphorus-co-ordination with-
out cleavage of the ring (a shift to a much lower field should
occur6 for ring opening). The fine structure for complex 1a
consists of a doublet [2J(PAPB) = 253.8 Hz] of broad triplets
[2J(PAPC) = 24 Hz], in agreement with the patterns observed for
Complex 2 appears to exist in solution in two isomeric forms,
both having equivalent phosphirene ligands, such as II and III
(although isomerism resulting from different relative confor-
mations of the two types of diazenide ligands22 cannot be ruled
out), as indicated by its 31P-{1H} NMR spectrum (CDCl3)
which consists of two similar sets of resonances, with a slight
predominance of one of them. In each set the resonance of
the phosphirene-phosphorus nuclei (2PA) occurs as a doublet
[2J(PAPB) = 13.8 Hz] at a chemical shift (δ Ϫ144.43 or Ϫ147.85
relative to H3PO4) that, as for complexes 1a and 1b, is typical
for phosphorus-co-ordination without ring rupture, whereas
the phosphine-phosphorus (PB) resonance is the expected
triplet (with the same coupling constant and half the intensity
of the above doublet) at δ 5.68 or 3.63.
2
the trans-phosphine [PB at δ Ϫ28.54 as a doublet, J(PBPA) =
2
253.1, of triplets, J(PBPC) = 17.5 Hz] and the cis-phosphines
2
[2PC at δ Ϫ28.39 as a triplet, J(PCPB) ≈ 2J(PCPA) ≈ 22 Hz]. A
more simplified 31P-{1H} NMR pattern is observed for 1b
(in CDCl3) in view of the lack of resolution of the cis couplings:
PA and PB appear simply as doublets [2J(PAPB) ≈ 240 Hz] (the
latter at δ Ϫ30.83), whereas 2PC resonate as a broad singlet at
δ Ϫ26.70.
The 1H NMR spectrum of complex 1a is also consistent
with the meridional arrangement of the phosphines, since the
resonance of the methyl protons of the unique phosphine
PB(CH3)2Ph is a doublet [2J(HPB) = 7.6 Hz] at δ 1.26, whereas
the resonance of the two trans PC(CH3)2Ph phosphines occurs
as two triplets (at δ 1.66 and 1.62), each of them as a result
of virtual coupling to the two PC nuclei [¹|2J(HPC) ϩ 4J(HPC)| ≈
¯
²
2.9 Hz]. Such a type of resonance pattern associated with
the meridional arrangement of PMe2Ph ligands has been
recognized17a,21 for other complexes.
The phenyldiazenide complex [ReBr3(NNPh)(PPh3)2] also
reacts with the phosphirene, in refluxing thf for 2 d, leading
to the formation of the orange complex [ReBr(NNPh)2-
Electrochemical studies
The anodic behaviours of complexes 1a, 1b and 2 were investi-
gated by cyclic voltammetry, at a platinum-wire electrode, in
0.2 M [NBu4][BF4]–CH2Cl2. Each of the dinitrogen complexes
(PPhCPh᎐CPh) (PPh )] 2, eqn. (2), which precipitated (ca. 20%
᎐
2
3
I
ox
presents a first reversible single-electron anodic wave at E
=
1/2
PPhCPh᎐CPh
0.19 (1a) or 0.27 (1b) V vs. SCE, assigned to the ReI → ReII
oxidation, which is followed by a second irreversible one, at
a higher potential(IIEpox = 1.25 1a or 1.35 V 1b) involving the
ReII → ReIII oxidation with N2 loss.
᎐
[ReBr3(NNPh)(PPh3)2]
thf, heat
[ReBr(NNPh) (PPhCPh᎐CPh) (PPh )] (2)
᎐
2
2
3
2
For mer-[ReCl(N )(PPhCPh᎐CPh)(PMePh ) ] 1b the value
᎐
2
2 3
yield) as the first product on concentration of the solution and
addition of pentane. From the mother-liquor it was possible to
isolate (in ca. the same yield) another product, already identi-
fied19 as [ReBr2(NNPh)2(PPh3)2], which does not contain any
phosphirene ligand.
I
ox
1/2
of E is identical to that observed for the parent complex
trans-[ReCl(N2)(PMePh2)4] (0.27 V, measured under identical
experimental conditions), whereas for the PMe2Ph analogues,
1a is oxidized at a higher potential than the parent N2 complex
(0.13 V), thus indicating that the phosphirene ligand behaves as
a net electron σ donor minus π-acceptor ligand identical to
PMePh2 but weaker than PMe2Ph. Hence, the values of the
The formation of these products is unexpected, involving the
formal replacement of a bromide by a diazenide ligand and, in
the case of 2, also metal reduction (possibly by PPh3 producing
Ph3PBr2) and further displacement of bromide and phosphine
by phosphirene. The mechanisms for these conversions are
unknown, but they conceivably occur via the formation of
bromide and diazenide bridges between two metal centres. The
starting diazenide complex [ReBr3(NNPh)(PPh3)2] is known19
to undergo reduction reactions with isocyanides to form
products of the type [ReBr2(NNPh)(CNR)(PPh3)2] (R =
Me or C6H4Cl-4), [ReBr2(NNPh)(CNMe)2(PPh3)] or [ReBr-
(NNPh)(CNMe)2(PPh3)2], but, in contrast to the present case,
there is no increase in the number of diazenide ligands.
The IR spectrum (KBr pellet) of [ReBr(NNPh)2-
23
24
electrochemical EL and PL parameters (which constitute a
measure of such a ligand character) for the phosphirene should
be identical to those of PMePh2, i.e. 0.37 V vs. NHE23 and
Ϫ0.43 V, respectively [the latter estimated from the observed23
linear correlation between those two parameters, i.e. PL = 1.17,
EL Ϫ0.86].
The measured values of the oxidation potential for com-
plexes 1a and 1b (0.43 and 0.52 V vs. NHE, respectively) are
in very good agreement with those (0.44 and 0.51 V vs. NHE,
respectively) predicted by using the Lever eqn. (3)23 in which
E = SM (ΣEL) ϩ IM (V vs. NHE)
(3)
(PPhCPh᎐CPh) (PPh )] 2 displays two bands at 1660 and 1560
᎐
2
3
cmϪ1, assigned to ν(NN) of the phenyldiazenide ligand, which
are within the range observed19 for related diazenide com-
plexes such as the parent one (1700 and 1575 cmϪ1) and
[ReCl(NNPh)2(PPh3)2] (1540 and 1510 cmϪ1) and the presence
of the diazenide ligands is confirmed by elemental analysis. It
seems likely that one NNPh presents a doubly bent geometry
ΣEL is the sum of the EL values for all the ligands, SM and IM are
dependent on the redox metal couple, spin state and stereo-
chemistry (SM = 0.76 and IM = Ϫ0.95 V vs. NHE for ReI/ReII
sites),23b thus confirming our estimate of the EL parameter
for the phosphirene ligand and the additive character of this
parameter for complexes 1a and 1b.
3756
J. Chem. Soc., Dalton Trans., 1999, 3755–3758

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Also in accord with this expression, the oxidation potentials
NHE (normal hydrogen electrode) were estimated by adding
of complexes 1a and 1b and related ones follow the order of
0.245 V to the corresponding ones quoted relative to SCE.
EL for the phosphorus variable ligand in the following way:
ox
trans-[ReCl(N2){P(OMe)3}4] (E = 0.42 V,18a EL = 0.42 V vs.
Syntheses
1/2
ox
NHE23a) > trans-[ReCl(N2)(PMePh2)4], 1b (E = 0.27 V, EL =
1/2
mer-[ReCl(N )(PPhCPh᎐CPh)(PMe Ph) ] 1a. The com-
0.37 V vs. NHE23a) or trans-[ReCl(N2)(Ph2PCH2CH2PPh2)2]
᎐
2
2
3
(E = 0.28 V,18a EL = 0.36 V vs. NHE23a) > 1a [E = 0.19 V,
ox
1/2
ox
1/2
pound PPhCPh᎐CPh (0.105 g, 0.366 mmol) was added to a
᎐
thf (13 cm3) solution of trans-[ReCl(N2)(PMe2Ph)4] (0.294 g,
0.366 mmol) and the system stirred for 3 d giving a pale orange
precipitate of complex 1 which was filtered off, washed with
thf–hexane and dried in vacuo. Further crops of product could
be obtained from the mother-liquor upon concentration and
addition of hexane (total ca. 0.17 g, 50% yield) (Found: C, 55.2;
EL (PMe2Ph) = 0.34 V vs. NHE,23a E (PPhCPh᎐CPh) = 0.37 V
᎐
L
ox
1/2
vs. NHE] > trans-[ReCl(N2)(PMe2Ph)4] (E = 0.13 V, EL = 0.34
V vs. NHE23a).
An increase in the oxidation potential of the two dinitrogen
complexes 1a and 1b corresponds to a decrease of the net
electron donor ability of the phosphorus ligands and is also
H, 5.1; N, 2.9. Calc. for C44H48ClN2P4Re: C, 55.6; H, 5.1;
N, 3.0%). IR (Nujol mull): 1944s, 1930s [ν(N᎐N), split due to
᎐
followed by an increase of ν(N᎐N) which reflects the lowering
᎐
᎐
᎐
of the π-electron release from the metal to the N2 ligand.
1
a solid state effect]. H NMR (CD2Cl2): δ 8.1–6.8 (m, 30 H,
I
ox
1/2
᎐
Moreover, the E and ν(N᎐N) data for complexes 1a and 1b
᎐
C6H5), 1.66 [t, ¹ |2J(HPC) ϩ 4J(HPC)| = 2.8, 6 H, PC(CH3)2Ph],
¯
²
fit reasonably the linear correlation between these parameters
recognised18a for a series of other rhenium() complexes with
the common Cl–Re–N2 axis.
1.62 [t, ¹ |2J(HPC) ϩ 4J(HPC)| = 2.9, 6 H, PC(CH3)2Ph] and
¯
²
2
1.26 [d, J(HPB) = 7.6 Hz, 6H, PB(CH3)2Ph]. 31P-{1H} NMR
(CD2Cl2): δ Ϫ28.39 [t, 2J(PCPB) ≈ 2J(PCPA) ≈ 22, 2PCMe2Ph],
Ϫ28.54 [dt, 2J(PBPA) = 253.1, 2J(PBPC) = 17.5, PBMe2Ph]
and Ϫ119.71 [dbrt, 2J(PAPB) = 253.8, 2J(PAPC) = 24 Hz,
The diazenide complex [ReBr(NNPh) (PPhCPh᎐CPh) -
᎐
2
2
(PPh3)] 2 also exhibits, by cyclic voltammetry, a single-electron
ox
1/2
reversible anodic wave which, however, occurs at E = 0.95 V
P PhCPhC᎐CPh].
᎐
A
vs. SCE, a value much higher than those of the above
ReI–dinitrogen complexes, in agreement with the higher metal
oxidation state for the former complex. In accord, the oxidation
potential of 2 is not so anodic as that exhibited by its parent
complex [ReBr3(NNPh)(PPh3)2], having a higher metal oxi-
dation state, which displays an irreversible oxidation wave at
Epox = 1.49 V vs. SCE.
The above electrochemical results, which are indicative
that the phosphirene as a ligand behaves as a net electron
donor/acceptor similar to PMePh2 and is also compatible
with N2 co-ordination, suggest that a novel phosphirene-based
nitrogen-fixation chemistry conceivably can be developed,
paralleling that known15 for dinitrogen–phosphine complexes.
mer-[ReCl(N )(PPhCPh᎐CPh)(PMePh ) ] 1b. The com-
᎐
2
2 3
pound PPhCPh᎐CPh (0.057 g, 0.20 mmol) was added to a
᎐
thf (40 cm3) solution of trans-[ReCl(N2)(PMePh2)4] (0.20 g,
0.20 mmol) and the solution stirred for 2 d. Concentration in
vacuo followed by addition of pentane led to the precipitation
of complex 1b as a dark yellow solid which was filtered off,
washed with thf–pentane and dried in vacuo. Further product
could be obtained from the mother-liquor upon concentration
and addition of pentane (total ca. 0.080 g, 40% yield). (Found:
C, 61.8; H, 4.4; N, 2.4. Calc. for C59H54ClN2P4Re: C, 62.3;
1
᎐
H, 4.8; N, 2.5%). IR (KBr pellet): 1940s [ν(N᎐N)]. H NMR
᎐
(CDCl3): δ 7.51–6.78 (m, 45 H, C6H5), 1.88 [s, br, 3 H,
PB(CH3)Ph2] and 1.65 [s, br, 6 H, 2PC(CH3)Ph2]. 31P-{1H}NMR
2
(CDCl3): δ Ϫ26.70 (s, br, 2PCMePh2), Ϫ30.83 [d, J(PBPA) ≈
Experimental
240, PBMePh2] and Ϫ131.10 [d, 2J(PAPB) ≈ 240 Hz,
Solvents were dried and degassed by using standard techniques.
All reactions were performed under an inert atmosphere
(N2). Triphenylphosphirene,25 trans-[ReCl(N2)L4] (L = PMe2Ph
or PMePh2)18b and [ReBr3(NNPh)(PPh3)2]19 were prepared
according to published methods.
P PhCPh᎐CPh].
A
᎐
[ReBr(NNPh) (PPhCPh᎐CPh) (PPh )] 2. The compound
᎐
2
2
3
PPhCPh᎐CPh (60 mg, 0.21 mmol) was added to a suspension
᎐
Infrared spectra were recorded on a Perkin-Elmer 683
spectrophotometer and NMR spectra on a Varian Unity 300
MHz or a Bruker AMX 500 MHz (or 80 MHz) spectrometer;
δ values are in ppm relative to SiMe4 (1H) or to H3PO4 (31P).
Abbreviations: s = singlet, d = doublet, t = triplet, br = broad,
dt = doublet of triplets, dbrt = doublet of broad triplets.
of [ReBr3(NNPh)(PPh3)2] (0.20 g, 0.19 mmol) in thf (60 cm3)
and the system heated to reflux during 2 d forming a dark
orange solution which was then concentrated in vacuo until
ca. 10 cm3. Pentane was added and complex 2 precipitated as
an orange solid which was filtered off, washed with a mixture
of thf and pentane, dried in vacuo and recrystallised from
CH2Cl2–Et2O (ca. 0.050 g, 20% yield) (Found: C, 63.1; H, 4.1;
N, 3.9. Calc. for C70H55BrN4P3Reؒ0.25CH2Cl2: C, 63.3; H, 4.2;
The electrochemical experiments were carried out either on
an EG&G PAR 173 potentiostat/galvanostat and an EG&G
PARC 175 Universal programmer or on an HI-TEK DT 2101
potentiostat/galvanostat and an HI-TEK PP RI waveform
generator. Cyclic voltammetry studies were undertaken in a
two-compartment three-electrode cell, at a platinum wire
working electrode, probed by a Luggin capillary connected to a
silver-wire pseudo-reference electrode; a platinum or tungsten
auxiliary electrode was employed. The first anodic wave in
the cyclic voltammograms of the complexes 1a and 1b has ∆Ep
of ca. 100 mV, ip(anodic)/ip(cathodic) close to one, and the
current function ipCϪ1νϪ1/2 (C = concentration, ν = scan rate)
without appreciable variation in the 50–1000 mV sϪ1 scan rate
range, thus following the usual criteria for a single-electron
reversible process. The oxidation potentials of the complexes
were measured by cyclic voltammetry in 0.2 mol dmϪ3
[NBu4][BF4]–CH2Cl2, and are quoted, unless stated otherwise,
relative to the SCE (saturated calomel electrode) by using
the [Fe(η5-C5H5)2]0/ϩ couple (0.55 V vs. SCE) as an internal
reference. The values of the oxidation potentials relative to
1
N, 4.2%). IR (KBr pellet): 1660m, 1560m [ν(NN)]. H NMR
(CDCl3): δ 7.71–6.64 (m, C6H5). 31P-{1H} NMR (CDCl3)
(2 isomers, the 31P resonances of the dominant one being given
in italics): δ 5.68 and 3.63 [t, 2J(PBPA) = 14, PBPh3], Ϫ144.43 and
2
Ϫ147.85 [d, J(P P ) = 14 Hz, P PhCPh᎐CPh]. Concentration
᎐
B
A
A
in vacuo of the mother-liquor, followed by addition of pentane,
led to precipitation of the known19 complex [ReBr2(NNPh)2-
(PPh3)2] as a red crystalline solid which was filtered off, washed
with Et2O and dried in vacuo (ca. 15% yield).
Acknowledgements
This work has been partially supported by the PRAXIS XXI
programme, the Junta Nacional de Investigação Científica e
Tecnológica (JNICT) and the Foundation for Science and
Technology (FCT) (Portugal), and the Treaty of Windsor pro-
gramme (The Portuguese Council of Rectors/The British
Council).
J. Chem. Soc., Dalton Trans., 1999, 3755–3758
3757

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Pombeiro, in New Trends in the Chemistry of Nitrogen Fixation,
eds. J. Chatt, L. M. Câmara Pina and R. L. Richards, Academic
Press, New York, 1980, ch. 6, p. 153; R. A. Henderson, G. J. Leigh
and C. J. Pickett, Adv. Inorg. Chem., 1983, 27, 197.
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