Bisphosphine monoxides with o-phenylene backbones in Pt, Pd and Fe complexes

Article abstract of DOI:10.1016/j.poly.2009.08.013

A new route to aromatic bisphosphine monoxides has been explored through ortholithiation of triphenylphosphine oxide and subsequent reaction of the lithiated intermediate with a range of alkyl and aryl phosphine chlorides. Routes to the known bisphosphine monoxide (BMPO) (o-C₆H₄){P(O)Ph₂}PPh₂ (2aO) and a range of new BPMOs of the type Ph₂P(O)(o-C₆H₄)PR₂ where R₂ = ⁱPr₂, Cy₂, Et₂ are described. Reaction of 2aO with MCl₂(cod) (M = Pd, Pt; cod = cyclooctadiene) gives products of the form [MCl(κ¹-P-2aO)(κ²-P,O-2aO)]⁺ Cl^- and MCl₂(κ²-P,O-2aO); the former exhibits fluxional behaviour which has been analysed by ¹⁹⁵Pt NMR and ³¹P variable-temperature NMR spectroscopy. The bidentate complex is not fluxional for either the Pd or the Pt example; the Pt complex PtCl₂(κ²-P,O-2aO) has been characterised by X-ray crystallography. By comparison of the product distribution seen by ³¹P NMR spectroscopy and ESI-MS it was established that the different coordination modes of 2aO result in quite different behaviour of the complexes when studied by ESI-MS; when the O is formally coordinated to the metal its ionisation efficiency is very low. Synthesis of Fe(CO)₄(2aO) confirmed the ability of the 2aO ligand to render a neutral complex with no alternative pathways for ionisation to be readily detected by ESI-MS.

Full text of DOI:10.1016/j.poly.2009.08.013

Polyhedron 29 (2010) 254–261
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Bisphosphine monoxides with o-phenylene backbones in Pt, Pd and Fe complexes
Nicola J. Farrer a,b, Robert McDonald c, Toria Piga b, J. Scott McIndoe b,
*
a Department of Chemistry, The University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK
b Department of Chemistry, The University of Victoria, P.O. Box 3065, Victoria, BC, Canada V8W 3V6
c X-ray Crystallography Laboratory, Department of Chemistry, University of Alberta, Edmonton, AB, Canada T6G 2G2
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 19 August 2009
A new route to aromatic bisphosphine monoxides has been explored through ortholithiation of triphen-
ylphosphine oxide and subsequent reaction of the lithiated intermediate with a range of alkyl and aryl
phosphine chlorides. Routes to the known bisphosphine monoxide (BMPO) (o-C6H4){P(O)Ph2}PPh2
(2aO) and a range of new BPMOs of the type Ph2P(O)(o-C6H4)PR2 where R2 = iPr2, Cy2, Et2 are described.
Keywords:
Reaction of 2aO with MCl2(cod) (M = Pd, Pt; cod = cyclooctadiene) gives products of the form [MCl(
2aO)(
2-P,O-2aO)]+ Clꢀ and MCl2( 2-P,O-2aO); the former exhibits fluxional behaviour which has been
analysed by 195Pt NMR and 31P variable-temperature NMR spectroscopy. The bidentate complex is not
fluxional for either the Pd or the Pt example; the Pt complex PtCl2(
2-P,O-2aO) has been characterised
j
1-P-
Phosphine ligands
Hemilabile ligands
Electrospray ionisation
Mass spectrometry
NMR spectroscopy
j
j
j
by X-ray crystallography. By comparison of the product distribution seen by 31P NMR spectroscopy
and ESI-MS it was established that the different coordination modes of 2aO result in quite different
behaviour of the complexes when studied by ESI-MS; when the O is formally coordinated to the metal
its ionisation efficiency is very low. Synthesis of Fe(CO)4(2aO) confirmed the ability of the 2aO ligand
to render a neutral complex with no alternative pathways for ionisation to be readily detected by ESI-MS.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
tity by hydrolysis of 1-Ph2@N(SiMe3)-2-(Ph2P)C6H4 [12] and in
good yield through the biphasic catalytic oxidation of dppbz [3].
Bisphosphine monoxides (BPMOs) are an important class of
hemilabile ligands [1]. The presence of both soft (P) and hard (O)
Lewis base centres in one molecule allows the ligand, when com-
plexed to a metal, to generate reactive, coordinatively unsaturated
species. Dissociation of the more weakly binding donor atom pro-
vides paths to transformations at the metal centre (e.g. oxidative
addition). BPMOs are capable of stabilising a wide range of transi-
tion metals, in both high and low oxidation states. The various
modes of coordination of a BPMO to a metal centre are shown in
Fig. 1 (along with some representative backbones), and vary
depending on the nature of the metal [2].
Numerous applications exist for complexes of BPMOs [3], such
as polymerisation [4], hydroformylation [5,6], hydrogenation [7],
olefin hydration [8], and sulfoxidation reactions [9]. Inclusion of
phosphine oxide functional groups often increases the water
solubility of ligands [10] making them ideal for recovery through
aqueous extraction or for catalytic transformations in water.
BPMOs with non-carbon backbones have also received attention,
particularly those of the form R2P–N(H)–P(O)R2 [11].
The ligand has found use in Ni-catalysed ethylene oligomerisation
[4,13]; the rigidity of the o-phenylene backbone of the phosphine
ligand pre-organises the ligand for chelation and contributes to a
high activation energy for b-elimination of the growing oligomer
from the metal centre – proceeding through a penta-coordinated
species – and hence to a high oligomerisation number.
There are two established synthetic routes to BPMOs [3]:
combination of a phosphine and a phosphine oxide fragment, or
selective oxidation of a bisphosphine. Coordination of a bisphos-
phine to a metal is commonly employed in selective oxidation
methods [14] and although phosphines of the type R2P(CH2)nPR2
(R = alkyl) may be selectively mono-oxidised by a ‘‘protonation fol-
lowed by oxidation” approach [15], aryl phosphines are less basic
and require alternative synthetic routes. Applications of BPMOs
with aromatic backbones are not well-developed, perhaps due to
the difficulties of preparing this type of ligand. Selective catalytic
mono-oxidation of dppm, dppe, dppp, dppb, dppbz, dppfc and BIN-
AP has been demonstrated, albeit by a method which requires prior
synthesis of the corresponding bisphosphine [16]. A recent and
promising report of monoreduction of a variety of bisphosphine
dioxides may provide improved access to BPMOs with aromatic
backbones [17], however, it is not possible to synthesise mixed
BPMOs in which the less reactive phosphine is oxidised (e.g.
R = Ar c.f. R = Et) by either of these routes.
The bisphosphine monoxide ligand (o-C6H4){P(O)Ph2}PPh2
(dppbzO, 2aO) has been synthesised adventitiously in small quan-
* Corresponding author. Fax: +1 250 721 7147.
E-mail address: mcindoe@uvic.ca (J.S. McIndoe).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.08.013

N.J. Farrer et al. / Polyhedron 29 (2010) 254–261
255
centre [37]. The regiochemistry of lithiation was confirmed
through NMR spectroscopy studies of the product mixture follow-
ing reaction with either MeI, Me3SiCl or D2O. Significantly, in addi-
tion to the mono-lithiated product, NMR spectroscopy and ESI-MS
both detected evidence for bis-lithiated products, albeit in rela-
tively low yield. Reaction of the lithiated 1O, 1OLi, with phosphine
chlorides (PR2Cl) proved straightforward and the steric restrictions
of the synthesis were tested through use of several different phos-
phine chlorides (Scheme 1).
BPMO products (2O) were obtained in the reactions with PPh2Cl
(70% conversion, based on phosphine oxide, by 31P NMR spectros-
copy, 2aO), PEt2Cl (62%, 2bO), PCy2Cl (18%, 2cO) and PiPr2Cl (3%,
2dO) and purified in the case of 2aO and 2bO. Conversion reduced
as the alkyl groups on the phosphine became larger; with no
detectable product following reaction with PtBu2Cl. In general,
31P NMR spectroscopy of 2O products revealed doublets
ꢂ30 ppm (OPPh2) and between ꢀ5 and ꢀ20 ppm (PR2) with a typ-
Fig. 1. Modes of coordination of bisphosphine monoxide ligands. Typical R groups
include Me, Ph, p-tolyl.
3
ical JPOP ꢃ 15 Hz (see Fig. S1).
Our interest in BPMOs is due to their potential for providing
high ionisation efficiencies in ESI-MS whilst coordinated to metal
complexes through the phosphine fragment. Phosphine oxides ion-
ise readily in an ESI source, typically through coordination of ions
such as H+ or Na+ at the oxygen lone pair [18]. This ‘‘electrospray
handle” would allow the complex to be readily detected in the
mass spectrum and provide a facile method for characterisation
[19]. We were also interested in preparing BPMOs on a reasonable
scale using cheap starting materials.
Consequently we investigated orthometallation of triphenyl-
phosphine oxide (OPPh3, 1O) and subsequent reaction with an
electrophile to provide orthosubstituted products [20]; with phos-
phine chlorides (PR2Cl) as electrophiles. Through subsequent
reduction this also provides a route to bisphosphines, a class of
compounds which find application in (enantioselective) [21] orga-
nometallic catalysis [22–25] and non-linear optics [26]. This paper
describes the results of these experiments, the detection of
byproducts, and the synthesis and characterisation of some new
metal–BPMO complexes and their behaviour under ESI-MS
conditions.
Small amounts of tris(phosphine) monoxides (TMPOs, 3aO,
3bO) were also obtained on reaction with PPh2Cl and PEt2Cl. These
gave 31P NMR spectroscopic signals as a triplet at low field (OPPh2)
and a doublet at high field (2ꢄPR2). In the formation of these
bissubstituted products the second lithiation is suggested to have
occurred on a different phenyl ring to the first, since lithiation is
anticipated to deactivate the first ring towards further substitution.
Double substitution provides a tantalising potential synthetic route
to novel ligands containing three phosphorus centres, however,
variation of the stoichiometry of the reaction between 1O and PhLi
did not significantly increase the yield of these products. Purifica-
tion of the BPMOs from unreacted 1O, PPh3 (presumably arising by
reaction between unreacted PhLi and PPh2Cl) was performed for
R = Ph and Et. The Et derivative was substantially more sensitive
to aerial oxidation than the Ph, and had to be handled with corre-
sponding care.
For R = Ph, a sample of 2aO and 3aO (0.4:1 ratio) containing no
dioxide products by 31P NMR spectroscopy showed dioxide prod-
ucts fairly prominently when analysed by ESI-MS (at +16 m/z above
the original, see Fig. 2). The fully oxidised dioxides 2aO2 and 3aO2
therefore demonstrate a significantly higher ionisation efficiency
in ESI-MS than the corresponding monoxides 2aO and 3aO, pre-
sumably since cation coordination is greatly enhanced by the che-
late effect. Both 2aO and 3aO ionise by forming [M+Na]+ ions in
preference to [M+Li]+ and by comparison to the ratio observed by
NMR, 2aO and 3aO demonstrate a reasonably similar ionisation
efficiency.
2. Results and discussion
2.1. Ligand syntheses
Aryl phosphine oxides react with organolithium [27] and orga-
nomagnesium [29] reagents on the aryl ring (typically ortho to the
phosphine oxide due to intramolecular Liꢁ ꢁ ꢁO interactions) and
also at the phosphorus atom. Suitable reaction conditions (reagent,
temperature, solvent) depend on the acidity of proton to be ab-
stracted, the presence of additional directing groups on the aryl
ring of the phosphine oxide and possible side-reactions due to
reaction at the phosphorus centre [28].
The sandwich compound Fe{g
5-C5H4P(O)Ph2}Cp can be
orthometallated [29] with MgBrꢁN(iPr)2ꢁ2THF [30,31]. We found
that this reaction fails for OPPh3, (1O). Lithium diisopropylamide
(LDA) [32,33], was also investigated without success. 1O does,
however, react with other alkyllithium and Grignard reagents
[34,35]; and were able to successfully ortholithiate 1O with PhLi
by a published method though careful adherence to experimental
detail [36]. The sparing solubility of 1O in cold diethyl ether made
stirring essential for the 72 h reaction, and maintenance of the tem-
perature in the range ꢀ25 °C P T P ꢀ35 °C was also important
since at temperatures much below this the titre of the PhLi solu-
tion dropped. It was necessary to use a lithium reagent bearing
the same aryl group (in this case Ph) as the phosphine oxide since
‘‘scrambling” of aryl groups may occur due to the competing
side-reaction of nucleophilic attack by the PhLi at the phosphorus
Scheme 1. General synthetic method for preparation of ortho-phenylene based bis-
and tris(phosphine)monoxides where R = Ph (2/3aO); Et (2/3bO); Cy (2cO); iPr
(2dO). The reaction was successful for all phosphine chlorides explored except
PtBu2Cl. Small amounts of TPMOs were observed for PPh2Cl (3aO) and PEt2Cl (3bO).

256
N.J. Farrer et al. / Polyhedron 29 (2010) 254–261
Fig. 2. ESI-MS (CH2Cl2/MeOH) of a 0.4:1 mixture of 2aO/3aO showing the nature of aggregation under soft ionisation conditions and the enhanced detection of dioxide
products (not seen by NMR) at 16 m/z units above the monoxide peak.
This synthetic method opens a route to relatively expensive bis-
phosphines such as 2a (dppbz); mono or bisphosphine oxides may
be reduced by methods typically employing hydrides such as diiso-
butylaluminium hydride (DIBAL-H) [38] or LiAlH4 [39,40] and si-
lanes such as phenylsilane [41] and trichlorosilane [42–45]. 2aO
is readily reduced to 2a with HSiCl3/P(OEt)3, as expected (Fig. S2).
45.4 ppm. On cooling the signal at 7.9 ppm split into two separate
resonances; at 198 K these were seen at 9.9 ppm (PC, 1JPtP 4524 Hz)
1
and 7.8 ppm (PE, JPtP 3736 Hz). The fluxionality is thought to be
due to the lability of the phosphine oxide groups, the symmetry
of the complex being broken when one phosphine oxide is me-
tal-bound and one is dissociated (Fig. 3; only the T range 240–
300 K is shown). NOESY and COSY experiments at 190 K confirmed
that PE and PF are within the same ligand, likewise with PC and PD.
The resonance corresponding to PD is seen at 32.8 ppm, similar to
the PO group of uncomplexed 2aO (29–32 ppm depending on sol-
vent) indicating that the interaction of this phosphine oxide group
with the metal centre is reduced at low temperature. At low tem-
2.2. Metal complexes
Catalysts employing phosphine ligands are commonly synthes-
ised in situ, from a catalyst precursor and the free ligand. We inves-
tigated the product distribution formed by reaction of 2aO with
both PtII and PdII species (Scheme 2) by NMR spectroscopy and
compared this with the product distribution detected by ESI-MS.
3
perature small (2JPOP 16 Hz, PE to PC; JPP 8 Hz, PE to PF) couplings
were seen for the j j
2-bound ligand (PE, PF); for the 1-bound ligand
(PC, PD) the corresponding coupling (2JPOP 16 Hz, PC to PE) was seen,
this modest value is consistent with a cis rather than trans arrange-
ment of phosphine ligands [48]. For complexes of 2aO we generally
observed that the 3JPP coupling of 2aO is lost if the phosphine oxide
group is not coordinated to the metal centre, presumably because
twisting of the ligand affects the through-bond coupling; a lack of
detectable 3JPP coupling between PC and PD at low temperature was
consistent with this. Essentially, cooling of the sample leads to the
hemilability of the ligand being frozen out. The mechanism by
which the exchange occurs is not known; the intermediate may in-
volve a three-coordinate [PtP2Cl]+ species, or a four-coordinate
[PtP2O2]2+ species.
2.3. Reaction with PtCl2(cod)
The 31P NMR spectrum at 294 K of the mixture formed by reac-
tion of PtCl2(cod) with 2aO revealed two main products (plus a
small amount of free ligand); [PtCl(
j j
1-P-2aO)( 2-P,O-2aO)]+ Clꢀ
and PtCl2(
j
2-P,O-2aO) in approximately a 4:1 ratio, with the for-
mer exhibiting fluxional behaviour. The reactivity seen for 2aO
shows similarities to the chelating ‘‘PAN” ligand 1-(dimethyl-
amino)-8-(diphenylphosphino)naphthalene, which reacts with
both PtII and PdII to form bidentate and monodentate complexes
[46], and BINAP(O) which has been shown to coordinate in both
j1 and j2 denticities to Pd [47].
2.4.2. 195Pt NMR spectroscopy
At 294 K a pseudo-triplet (doublet of doublets) was seen at
2.4. [PtCl(j j
1-P-2aO)( 2-P,O-2aO)]+ Clꢀ
ꢀ3971 ppm corresponding to [PtCl(
j j
1-P-2aO)( 2-P,O-2aO)]+ Clꢀ
(see Fig. S2), consistent with reports of similar compounds [49].
2.4.1. 31P NMR spectroscopy
This relates to a Pt species bonded to two nearly equivalent phos-
At ambient temperature the coordinated phosphine groups are
phine ligands at room temperature; the 1JPtP of 4159 Hz correlating
equivalent, with a single resonance at 7.9 ppm with 195Pt satellites
1
well with the corresponding PA JPPt coupling of 4152 Hz in the 31
P
1
and JPtP of 4152 Hz. The two phosphine oxide groups (PB) ex-
NMR spectrum. These values are larger than the typical range for
1JPt–P couplings in trans complexes (2400–3000 Hz) [50] reflecting
the increased bond strength of the Pt–P bonds in the cis complex
[51,52].
changed rapidly, corresponding to a broad resonance at 35.4–
2.4.3. PtCl2(
PtCl2(
2-P,O-2aO) was not fluxional over the temperature range
j
2-P,O-2aO)
j
studied (190–300 K). Two doublets (PX, PY) were seen in the 31P
NMR spectrum, the coordinated phosphine PX showing 195Pt satel-
lites (1JPPt 3770 Hz). The 195Pt NMR spectrum showed a corre-
sponding doublet at ꢀ3085 ppm (1JPtP = 3771 Hz) (Fig. S3)
indicating that only one BPMO ligand was attached to the metal
centre.
Scheme 2. Routes to metal complexes of the bisphosphine monoxide ligand 2aO.

N.J. Farrer et al. / Polyhedron 29 (2010) 254–261
257
Fig. 3. Variable temperature 31P NMR (202 MHz) spectra of PtCl2( 2-2aO) and [PtCl2( 1-P-2aO)( 2-2aO)]+ Clꢀ. PC and PE coalesce into PA at 262 K, and PD and PF coalesce into
j j j
PB at 292 K.
Fig. 4. ESI-MS (CH2Cl2/MeCN) of reaction between PtCl2(cod) and 2aO. The inset spectrum shows the overlaid experimental and calculated isotope patterns for [PtCl(2aO)2]+.
2.5. Mass spectrometric analysis of Pt complexes
present at low concentration and hence not detected by NMR spec-
troscopy, or which may form under the conditions of electrospray
ionisation. A reasonable dimeric structure can be envisaged for
[Pt2Cl2(2aO)3]2+, in which the chloride ligands bridge between
the two Pt centres.
A primary motivation for this work was to establish the perfor-
mance of BPMOs in improving ESI-MS analysis of metal complexes.
We investigated the ability of the phosphine oxide to associate
with charged species when the PO is pendant (
to a metal centre (
2) and also the competition between ionisation
through cation coordination and alternative ionisation pathways.
ESI-MS analysis of the mixture of [PtCl(
1-P-2aO)( 2-P,O-2aO)]+
Clꢀ and PtCl2( 2-P,O-2aO) showed derivatives of the former only
j
1) or coordinated
j
2.6. Fragmentation studies
j
j
MS/MS fragmentation pathways were studied using EDESI-MS/
MS. This involves collecting spectra over a wide range of collision
energies and combining all the results into a single map in which
ion intensity is contoured (see Fig. S5 for example) [55]. Such maps
make interpretation of complex fragmentation pathways consider-
ably easier [56]. Product ions formed by fragmentation of the most
abundant ions, [Pt2Cl2(2aO)3]2+, [Pt(2aO)2]2+ and [PtCl(2aO)2]+, are
shown in Fig. 5.
j
(Fig. 4), as the complex is ionised in solution. The cation
[PtCl(2aO)2]+ (MS gives no information on the mode of bonding
of the ligand) is observed, as is the dication [Pt(2aO)2]2+ – presum-
ably generated through chloride loss from the monocation during
the electrospray ionisation process. Halide loss to form [MꢀX]+
or [Mꢀ2X]2+ ions is a well-documented ionisation pathway for me-
tal halide complexes [53,54]. In this case we suggest that the addi-
tional pendant phosphine oxide group is responsible for facilitating
chloride loss through intramolecular stabilization of this coordin-
2.7. Fragmentation of [PtCl(2aO)2]+
atively unsaturated species. In comparison, the neutral PtCl2(j2
-
Fragmentation of [PtCl(2aO)2]+ involves decomposition by three
different pathways (Fig. 5a). Transfer of Phꢀ from the phosphine li-
gand to the platinum complex generates the charged product ion
[2aOꢀ Ph]+ at 385.1 m/z in the lowest energy fragmentation path-
way. Alternatively, loss of HCl from the precursor complex gives
[Pt(2aO)2ꢀH]+ at 1119.0 m/z. The highest energy fragmentation
of [PtCl(2aO)2]+ involves loss of an uncharged 2aO ligand to give
P,O-2aO) is only barely detected (0.2% intensity) as an [M+Na]+
ion at 751.00 m/z; no [MꢀCl]+ ion was observed for this compound.
The only other sodiated ion present was the free ligand, [2aO+Na]+,
whose appearance is consistent with the 31P NMR spectroscopic re-
sults. The dicationic peak at 923.89 m/z is assignable to
[Pt2Cl2(2aO)3]2+ (confirmed by MS/MS), a species that is probably

258
N.J. Farrer et al. / Polyhedron 29 (2010) 254–261
Fig. 5. MS/MS of (a) [PtCl(2aO)2]+, (b) [Pt(2aO)2]2+ and (c) [Pt2Cl2(2aO)3]2+ species seen in the ESI-MS spectrum.
[PtCl(2aO)]+ at 693.0 m/z, rapidly followed by loss of HCl to
give [Pt(2aO)ꢀH]+ at 656.0 m/z. This latter species fragments
further at a higher collision energy through sequential loss of
two Phꢅ groups and POꢅ to give the last detectable fragment
[Pt(2aO)ꢀPOPh2H]+ at 454.0 m/z. As cations do not commonly frag-
ment through anion loss to form dications, it is unsuprising that
fragmentation of [PtCl(2aO)2]+ does not produce [Pt(2aO)2]2+
.
2.8. Fragmentation of [Pt(2aO)2]2+
The dication [Pt(2aO)2]2+ detected under MS/MS conditions at
560.1 m/z fragments to produce two singly charged product ions
by transfer of a phenyl group from one ligand to Pt to form
[Pt(2aO)+Ph]+ at 734.0 m/z and elimination of the phosphonium
ion [2aOꢀ Ph]+, seen at 385.1 m/z (Fig. 5b). This fragmentation
pathway is outlined pictorially in Fig. 6.
At higher collision energies minor fragments are seen which
correspond to the loss of a neutral ligand plus smaller fragments;
these species are seen: [Pt(2aOꢀ 2Ph)]+, 502.0 m/z; [Pt(2aO)ꢀPh]+,
580.0 m/z; and [Pt(2aO)ꢀH]+, 656.0 m/z. The absence in the MS/MS
spectrum of species such as [2aO+H]+ and [2aO+Ph]+ indicate that
the neutral ligand and the smaller fragments (H+, Ph+) are lost
separately.
Fig. 6. Putative structure of the [Pt(2aO)2]2+ species (the cis version is equally
likely) and its primary fragmentation pathway.
divides into two monocations, [PtCl(2aO)2]+ and [PtCl(2aO)]+ each
retaining one chloride ligand. Both pathways are consistent with
the dimeric structure postulated for [Pt2Cl2(2aO)3]2+ which is
shown in Fig. 7.
2.9. Fragmentation of [Pt2Cl2(2aO)3]2+
2.10. X-ray crystallography
MS/MS of [Pt2Cl2(2aO)3]2+ reveals two fragmentation pathways
(Fig. 5c). The major route (pathway A) involves loss of neutral
PtCl2(2aO) to form [Pt(2aO)2]2+, and in the other (B), the dication
Yellow crystals grown by vapour diffusion (pentane/CH2Cl2)
were characterised by single-crystal X-ray diffraction and were

N.J. Farrer et al. / Polyhedron 29 (2010) 254–261
259
coordinating the oxygen more strongly, favouring the formally
bound PdCl2(
2-P,O-2aO) complex, whereas the softer Pt favours
coordination of the two P groups in PtCl2(
1-P-2aO)2. The regio-
chemistry of [PdCl(
1-P-2aO)( 2-P,O-2aO)]+ Clꢀ was assigned as
j
j
j
j
the P donors coordinating cis rather than trans based on the modest
2JPP coupling of 23 Hz at low temperature. A comprehensive review
of the literature places 2JPP cis couplings for PdII bisphosphine com-
plexes in the range ꢀ25 to +80 Hz [58], and trans couplings in the
range +441 to +1269 Hz [59]. Since the behaviour for Pd is reason-
ably similar to that observed for Pt, representative VT 31P NMR
spectral and ESI-MS data for the Pd derivatives are shown in the
supporting information.
Despite the differences in speciation seen by 31P NMR spectros-
copy for Pd vs Pt on complexation of 2aO, the ESI-MS speciation ap-
Fig. 7. Putative structure of the [Pt2Cl2(2aO)3]2+ species and its fragmentation
pathways.
peared largely the same (Fig. S4). The PdCl2(j
2-P,O-2aO) species
was – as in the Pt case – only weakly represented in the ESI-MS
(as the [M+Na]+ adduct) despite being shown by 31P NMR spectros-
copy to be the major product formed.
2.12. Fe(CO)4(
j
1-P-2aO)
Fe(CO)4(
j
1-P-2aO), which has no overall charge and which does
not possess an ionisation pathway (such as loss of Clꢀ which was
possible for the Pt and Pd species) except for coordination of a po-
sitive ion (e.g. Li+, Na+, H+) through the phosphine oxide, was pre-
pared from 2aO and Fe2(CO)9. 31P NMR spectroscopy showed that
the complex was not fluxional, and furthermore, the chemical shift
of the phosphine oxide group (dP: 29.9 ppm) is similar to that of the
free ligand suggesting that it is completely uninvolved with the
metal centre. By ESI-MS, Fe(CO)4(
j
1-P-2aO) was seen exclusively
+
as [M+M0]+ where M0 = Na+ or H+; Li+ and NH4 adducts were not
detected. MS/MS analysis showed the sequential loss of CO ligands
with a competing fragmentation pathway involving loss of Fe(CO)4
to leave just the charged ligand [2aO+Na]+ at 485 m/z (Fig. S6). The
ESI-MS also shows evidence for higher aggregates such as Fe2(-
CO)4(2aO)2 that are not observed in the 31P NMR spectra; possibly
they are only present in trace amounts.
Fig. 8. ORTEP plot of PtClX(
j
2-P,O-2aO)ꢁCH2Cl2 (X = Cl, 70%; Br, 30%). CH2Cl2 of
crystallisation not shown, non-hydrogen atoms are represented by Gaussian
ellipsoids at the 50% probability level. Hydrogen atoms are shown with arbitrarily
small thermal parameters. Selected bond lengths (Å): Pt–Cl(1) 2.3567(8); Pt–Cl(2)
2.269(10); Pt–P(1) 2.2147(7); Pt–O 2.060(2); P(1)–C(1) 1.839(3); P(2)–O 1.521(2);
P(2)–C(2) 1.801(3); C(1)–C(2) 1.402(4). Selected bond angles (°): Cl(1)–Pt–Cl(2)
91.4(4); Cl(1)–Pt–O 85.74(6); Cl(2)–Pt–P(1) 87.1(4); Cl(2)–Pt–O 176.2(4); P(1)–Pt–
3. Experimental
O
95.74(6); Pt–P(1)–C(1) 116.05(10); O–P(2)–C(2) 111.95(13); Pt–O–P(2)
3.1. General
116.15(11); P(1)–C(1)–C(2) 124.4(2); P(2)–C(2)–C(1) 122.5(2).
Dry, reagent grade solvents were obtained by distillation or
from a solvent purification system. Reagents were purchased from
Strem and Aldrich and used without further purification. Reactions
were performed under nitrogen using standard Schlenk tech-
niques. Silica columns were loaded and run under nitrogen. Elec-
shown to consist of the monosubstituted product PtCl2(j
2-P,O-
2aO) co-crystallised 1:1 with CH2Cl2 solvent (CCDC 699942). X-
ray crystallographic parameters for PtCl2(
2-P,O-2aO) are given
in Table S1. The molecular structure of [PtCl2(
2-P,O-2aO)] is given
j
j
trospray ionisation mass spectra were collected using
a
in Fig. 8 along with key bond lengths and angles. The Pt site trans to
oxygen contained a mixture of Cl and Br (70% and 30%, respec-
tively; the Br is presumed to originate from the PhLi/LiBr solution
used in the ligand synthesis). Coordination of 2aO to Pt in a biden-
tate fashion does not impose a great deal of strain on the 2aO
ligand, the torsion angle of ꢀ0.83° P(2)–C(2)–C(1)–P(1) indicating
that the central benzene ring is essentially undistorted and the bite
angle at Pt is ꢂ96°. Pt–P (2.2147 Å) and Pt–O (2.060 Å) bond dis-
tances are consistent with similar structures [57].
Micromass QTof micro instrument (cone voltage 15 V, nebuliser
tip 3100 V, nitrogen desolvation gas 80 °C). Data collection was
carried out in MCA mode for MS spectra and continuum mode
for MS/MS and EDESI spectra. NMR spectra were recorded on the
following Bruker spectrometers: AV-500, AC-300 or AMX-360.
Chemical shifts are quoted in d (ppm) referenced to residual sol-
vent resonances (1H, 13C) or external references of 85% aqueous
H3PO4 (
31P). Heteronuclear NMR spectra were run with non-deu-
terated solvents and an internal sealed glass capillary (a lock stick)
containing D2O allowing samples to be easily made up in a glove-
box. Assignments were assisted by 1H{31P}, 31P and 1H NOESY and
COSY experiments, 13C-DEPT, 1H–13C-HSQC and 1H–13C-HMBC
NMR experiments. 13C NMR assignments were consolidated by
running experiments on AV-500 and AC-300 instruments and com-
paring 31P–13C coupling. Full assignment of 13C spectra was not
possible in all cases due to the complexity of the 31P–13C coupling.
A 5 s delay was used on 31P NMR spectra, despite this it was
2.11. Reaction with PdCl2(cod)
The reaction between 2aO and PdCl2(cod) gave both
2-products, although in contrast to the Pt derivatives, PdCl2(j2
P,O-2aO) was the major product (2:1 ratio) compared to the
bissubstituted fluxional complex [PdCl(
1-P-2aO)( 2-P,O-2aO)]+
Clꢀ. This product selection may be due to the slightly harder Pd
j
1- and
j
-
j
j

260
N.J. Farrer et al. / Polyhedron 29 (2010) 254–261
observed that there was a small consistent and reproducible inte-
gration error, with PR3 groups being slightly underrepresented
compared to P(O)R3. Melting points were recorded on a Gallenk-
amp MPA and are uncorrected. IR spectra were recorded using a
solution cell on a Perkin–Elmer 1000 FTIR spectrometer. X-ray
3.1.5. [PtCl(j j j
1-P-2aO)( 2-P,O-2aO)]+ Cl– and PtCl2( 2-P,O-2aO)
PtCl2(cod) (0.0106 g, 0.028 mmol) [63] and 2aO (0.026 g,
0.06 mmol, 2 eq.) were stirred in CH2Cl2 (3 ml) for 2 h. Pentane
was added and the solution was left overnight at 5 °C, forming a
microcrystalline yellow precipitate. The product distribution in
crystallography data for PtClX(
j
2-P,O-2aO)ꢁCH2Cl2 was collected
the reaction solution consisted of [PtCl(j j
1-P-2aO)( 2-P,O-2aO)]+
in the X-ray Laboratory of the University of Alberta Chemistry
j
Clꢀ and PtCl2( 2-P,O-2aO) in a 4:1 ratio (as determined by 31P
Department. Measurements were made at ꢀ80 °C on a Bruker
NMR).
PLATFORM/SMART 1000 CCD diffractometer with Mo Ka radiation
3.1.6. [PdCl(j j j
1-P-2aO)( 2-P,O-2aO)]+ Cl– and PdCl2( 2-P,O-2aO)
(k = 0.71073 Å). The structure was solved by direct methods and
unit cell parameters were obtained from full-matrix least-squares
refinement on F2. All fully oxidised phosphines (including 1O) were
obtained through oxidation with H2O2 [60].
A CH2Cl2 (3 ml) solution of PdCl2(cod) (0.035 g, 0.04 mmol) and
2aO (0.034 g, 0.074 mmol) was stirred at 23 °C for 2 h. For 31P VT
NMR samples an aliquot of the reaction mixture was taken after
2 h and CD2Cl2 added as lock. If the original solution was left to
sit overnight an orange/yellow precipitate formed which was iso-
lated by suction filtration (0.038 g), washed with acetone and
recrystallised from CH2Cl2/pentane by vapour diffusion. Yellow
prisms formed, but were of insufficient quality for X-ray diffraction
3.1.1. Lithiation of triphenylphosphine oxide (1O)
An ethereal phenyllithium solution [61,62] (0.6–0.9 M) was
cooled to ꢀ25 °C using an acetone/CO2 bath. To this solution 1O
(1eq.) was added using a solid-addition device. The brown suspen-
sion was stirred in a freezer (ꢀ25 °C) for 72 h to give an opaque or-
ange solution of 1OLi.
studies. The products formed in the ratio 1:0.54 for [PdCl(
j
1-P-
2aO)(
j
2-P,O-2aO)]+ Clꢀ:PdCl2( 1-P-2aO)2 (as determined by 31P
j
NMR).
3.1.2. Reaction of 1OLi with PPh2Cl; isolation of 2aO and 3aO;
oxidation of 2aO to 2aO2
3.1.6.1. Fe(CO)4(j
1-P-2aO). Fe2(CO)9 (0.03 g, 0.08 mmol) was mea-
sured into a 2-neck flask (glovebox), stoppered and removed. Dry
THF was added (1.5 ml) followed by 2aO (0.037 g, 0.08 mmol) in
dry THF (0.5 ml). The resulting solution was heated at 60 °C for
2.5 h with stirring, followed by cooling and removal of the solvent
to give a red residue. Under inert atmosphere the residue was ex-
tracted with dry hexane (3 ml), which was then filtered to give a
PPh2Cl (2.9 ml, 15.7 mmol) was added to a stirred solution of
1OLi (23.5 ml, 0.65 M, 15.3 mmol) at ꢀ25 °C and the solution al-
lowed to warm to 23 °C overnight with stirring. The resulting yel-
low solution/cream precipitate consisted of a mixture of 2aO and
3aO. THF (10 ml) and 1,2-dichloroethane (30 ml) were added to
dissolve the product. The solution was washed (H2O, 2 ꢄ 20 ml),
NaCl(aq) (2 ꢄ 20 ml) and the solvent removed before redissolving
the residue in minimal CH2Cl2 and purifying by silica column chro-
matography (2:1 CH2Cl2/AcOEt). 2aO was isolated as white foamy
solid Rf 0.48 (yield 3.03 g, 6.5 mmol, 47%), and 3aO as a white solid;
Rf 0.68 (yield 0.30 g, 0.46 mmol, 3%). 2aO was oxidised with H2O2
to give 2aO2 which was isolated by precipitation from CH2Cl2/pen-
tane. See supporting information for assignment of structures 2aO
and 3aO and spectroscopic details.
bright yellow solution of Fe(CO)4(j
1-P-2aO) and unreacted 2aO.
4. Conclusions
A route to a class of known and novel BPMOs via ortholithiation
of triphenylphosphine oxide has been presented. Although only
modest conversions have been reported, and steric factors are
clearly an important limitation, the method involves cheap, readily
available materials and can be carried out on an appreciable scale
with sterically undemanding dialkylhalophosphines to access new
and known BPMOs, including those in which the least reactive
phosphine is the oxidised group. Under mass spectrometric condi-
tions the BPMO ligand 2aO shows high affinity for H+ and Na+; the
particular preference of one or the other is thought to depend on
cation availability in the instrument. 31P NMR spectroscopy of
the reaction of 2aO with PtCl2(cod) and PdCl2(cod) reveals that a
3.1.3. Reduction of 2aO to 2a
A pressure tube containing 2aO (0.092 g, 0.2 mmol) was evacu-
ated and placed under N2 (conducted behind a blast shield). THF
(3 ml) and toluene (3 ml) were added and the solid stirred to dis-
solution. The solution was freeze-pump-thaw degassed (6 cycles).
Triethylphosphite (0.33 ml, 2.0 mmol) and HSiCl3 (0.8 ml, 8 mmol)
were added with stirring. The solution was frozen, the headspace
evacuated and the tube sealed. The solution heated to 100 °C and
stirred (88 h). Ether (10 ml) was added and an aliquot of the solu-
tion was filtered and analysed by 31P NMR. The solvent was re-
moved and replaced with CDCl3 for further analysis.
j1 j2 fluxional mode of coordination is favoured for Pt, whereas
/
a
j2 non-fluxional mode is favoured for Pd. In Fe(CO)4( 1-P-2aO)
j
the phosphine oxide appears to be uninvolved with the Fe0 metal
centre. Detection of metal-bound 2aO species by ESI-MS depends
largely on the availability of the phosphine oxide lone pair for cat-
ion coordination; when bound in a j j
2-mode e.g. MCl2( 2-P,O-2aO)
3.1.4. Reaction of 1OLi with PR2Cl; oxidation of 2bO to 2bO2
(M = Pd, Pt) the complexes are only faintly detected by ESI-MS
(even when, in the case of Pd, the complex is the major product
of the reaction) suggesting that the lone pairs of the phosphine
oxide are significantly involved in bonding to the metal centre.
PEt2Cl (0.3 ml, 0.31 g, 2.5 mmol) was measured into a flask in
the glovebox, stoppered and removed. Dry ether (10 ml) was added
and the temperature reduced to ꢀ78 °C. The solution was stirred
and an ethereal solution of 1OLi was added (2.5 ml, 2.3 mmol).
The solution was allowed to warm to room temperature overnight
with stirring (31P NMR yield, 2bO: 62%, 3bO: 6%). The solvent was
removed under vacuum and the residue extracted with fluoroben-
zene (3 ꢄ 5 ml). The solvent was removed and the residue washed
with hexane (1 ꢄ 5 ml) and then extracted with toluene (4 ꢄ 5 ml).
The highly air-sensitive product 2bO crystallised as a white solid
(0.25 g, 0.68 mmol, 30%). 2bO and 3bO were oxidised with H2O2
to 2bO2 and 3bO3, respectively. 2cO and 2dO were prepared in
analogous fashion in yields of 18% and 3%, respectively.
Analysis of the X-ray crystallographic data of PtCl2(j
2-P,O-2aO)
shows the Pt–O bond distance (2.060 Å) to be comparable with
that of other Pt-bound phosphine oxide ligands. Promotion of Clꢀ
loss as demonstrated by 2aO in
j
1-bound complexes show it to
be an ideal candidate for reversible stabilisation of a metal centre
through the phosphine oxide group, a feature which is highly
desirable for catalytic transformations. The affinity of the PO group
for cations is very low when formally
j
2-bound, although it was
still possible to detect these complexes at low abundance by ESI-
MS.

N.J. Farrer et al. / Polyhedron 29 (2010) 254–261
261
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Acknowledgments
JSM thanks NSERC, CFI, BCKDF, and the University of Victoria for
instrumentation and operational funding, and the EPSRC for a First
Grant. NF thanks Pieter Bruijnincx for helpful discussions. We
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31P NMR spectra, ESI-MS, EDESI-MS, ESI-MS/MS. Supplementary
data associated with this article can be found, in the online version,
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