The synthesis of, and characterization of the dynamic processes occurring in Pd(ii) chelate complexes of 2-pyridyldiphenylphosphine

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www.rsc.org/dalton | Dalton Transactions

The synthesis of, and characterization of the dynamic processes occurring in

Pd(^II) chelate complexes of 2-pyridyldiphenylphosphine†

Jianke Liu, Chacko Jacob, Kelly J. Sheridan, Firas Al-Mosule, Brian T. Heaton, Jonathan A. Iggo,*

Mark Matthews, Jeremie Pelletier, Robin Whyman, Jamie F. Bickley and Alexander Steiner

Received 3rd September 2009, Accepted 4th June 2010

First published as an Advance Article on the web 26th July 2010

DOI: 10.1039/b918162h

Pd(II) complexes in which 2-pyridyldiphenylphosphine (Ph₂Ppy) chelates the Pd(II) centre have been

prepared and characterized by multinuclear NMR spectroscopy and by X-ray crystallographic analysis.

1

2

1

trans-[Pd(k -Ph₂Ppy)₂Cl₂] is transformed into [Pd(k -Ph₂Ppy)(k -Ph₂Ppy)Cl]Cl by the addition of a few

2

1

drops of methanol to dichloromethane solutions, and into [Pd(k -Ph₂Ppy)(k -Ph₂Ppy)Cl]X by addition

-

1

of AgX or TlX, (X = BF₄, CF₃SO₃or MeSO₃). [Pd(k -Ph₂Ppy)₂(p-benzoquinone)] can be

2

1

transformed into [Pd(k -Ph₂Ppy)(k -Ph₂Ppy)(MeSO₃)][MeSO₃] by the addition of two equivalents of

2

1

MeSO₃H. Addition of further MeSO₃H affords [Pd(k -Ph₂Ppy)(k -Ph₂PpyH)(MeSO₃)][MeSO₃]₂.

Addition of two equivalents of CF₃SO₃H, MeSO₃H or CF₃CO₂H and two equivalents of Ph₂Ppy to

2

1

-

[Pd(OAc)₂] in CH₂Cl₂or CH₂Cl₂–MeOH affords [Pd(k -Ph₂Ppy)(k -Ph₂Ppy)X]X, (X = CF₃SO₃,

-

MeSO₃or CF₃CO₂), however addition of two equivalents of HBF₄·Et₂O affords a different complex,

2

tentatively formulated as [Pd(k -Ph₂Ppy)₂]X₂. Addition of excess acid results in the clean formation of

2

1

[Pd(k -Ph₂Ppy)(k -Ph₂PpyH)(X)]X₂. In methanol, addition of MeSO₃H and three equivalents of

Ph₂Ppy to [Pd(OAc)₂] affords [Pd(k -Ph₂Ppy)(k -Ph₂Ppy)₂][MeSO₃]₂as the principal Pd-phosphine

2

1

complex. The ﬂuxional processes occuring in these complexes and in [Pd (k -Ph₂Ppy)₃Cl]X, (X = Cl,

OTf) and the potential for hemilability of the Ph₂Ppy ligand has been investigated by

variable-temperature NMR. The activation entropy and enthalpy for the regiospeciﬁc ﬂuxional

2

1

processes occuring in [Pd(k -Ph₂Ppy)(k -Ph₂Ppy)₂][MeSO₃]₂have been determined and are in the range

-10 to -30 J mol^-1K^-1and ca. 30 kJ mol^-1respectively, consistent with associative pathways being

followed. The observed regioselectivities of the exchanges are attributed to the constraints imposed by

microscopic reversibility and the small bite angle of the Ph₂Ppy ligand. X-Ray crystal structure

1

2

1

determinations of trans-[Pd(k -Ph₂Ppy)₂Cl₂], [Pd(k -Ph₂Ppy)(k -Ph₂Ppy)Cl][BF₄],

1

[Pd(k -Ph₂Ppy)₂(p-benzoquinone)], trans-[Pd(k -Ph₂PpyH)₂Cl₂][MeSO₃]₂, and [Pd(k -Ph₂Ppy)₃Cl](Cl)

2

1

are reported. In [Pd(k -Ph₂Ppy)(k -Ph₂Ppy)Cl][BF₄] a donor–acceptor interaction is seen between the

pyridyl-N of the monodentate Ph₂Ppy ligand and the phosphorus of the chelating Ph₂Ppy resulting in a

trigonal bipyramidal geometry at this phosphorus.

of coordinating temporarily to a transition-metal centre in the

Introduction

absence of substrate, but that in the presence of substrate, can

be readily displaced to give a metal–substrate complex, offers the

potential for effective stabilization of the metal centre without

compromising its activity. The substitutionally inert portion of

these ligands can be used to tune the selectivity of the transition-

metal centre and the presence of the labile donor group may

allow this part of the ligand to play a dual role, both as a donor

group when required and via a “secondary interaction” such as

a proton transfer agent when decoordinated from the metal.^6,7

Hemilabile ligands have indeed been successfully employed in a

range of catalytic reactions, including methanol carbonylation,

hydrogenation, hydroformylation and polymerisation of ethene

with CO.⁴A common combination of donor functionalities in

such ligands is a phosphine with a nitrogen donor; such ligands

have shown promise in many catalytic reactions.^2,8–15For example,

Drent and co-workers,^16,17reported Pd systems incorporating

pyridylphosphine ligands which show activities three or more

orders of magnitude greater than analogous phosphine based

systems even under considerably milder conditions (318 vs.

388 K).^7,17Drent attributed this remarkable increase in activity

The coordination chemistry of chelating ligands containing mixed

functionalities at transition-metal centres is an extremely active

area of research since the presence of two different donor groups

should allow tuning of the coordination chemistry and hence

functionality of the resulting complexes.^1–5For example, the

catalytic utility of hemilabile ligands has received considerable

attention since the presence of a weakly chelating group, capable

Department of Chemistry, Donnan and Robert Robinson Laboratories,

University of Liverpool, P. O. Box. 147, Liverpool, UK, L69 7ZD. E-mail:

iggo@liv.ac.uk; Fax: 44 (0)151 794 3588; Tel: 44 (0)151 794 3538

† Electronic supplementary information (ESI) available: NMR spectra

of the complexes and simulations of the variable-temperature data.

2

Eyring and Arrhenius plots for the dynamic processes in [Pd(k -

1

2

1

2

Ph₂Ppy)(k -Ph₂Ppy)Cl](Cl), [Pd(k -Ph₂Ppy)(k -Ph₂Ppy)Cl][OTf], [Pd(k -

1

Ph₂Ppy)(k -Ph₂Ppy)₂][MeSO₃]₂and [Pd(k -Ph₂Ppy)₃Cl][CF₃SO₃]. Crystal

1

data and structure reﬁnement for trans-[Pd(k -Ph₂Ppy)₂Cl₂], trans-[Pd(k -

Ph₂PpyH⁺)₂Cl₂][MeSO₃]₂, [Pd(k -Ph₂Ppy)(k -Ph₂Ppy)Cl][BF₄], [Pd(k -

2

1

Ph₂Ppy)₂(benzoquinone)] and [Pd(k -Ph₂Ppy)₃Cl](Cl). CCDC reference

numbers 746623–746627. For ESI and crystallographic data in CIF or

other electronic format see DOI: 10.1039/b918162h

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1

the ³¹P{ H} NMR spectrum (Supplementary Fig. 1, ESI†). The

two high frequency resonances, d(P) 29.0 and 22.8 ppm, indicate

Ph₂Ppy ligands bound through phosphorus only (cf. that of the

free ligand d(P) -4.5 ppm),³whilst the extremely low frequency

chemical shift of the third resonance, d(P) -61.6 ppm, is character-

istic for this ligand in a strained, four-membered chelate ring.⁴¹The

absence of coupling indicates one type of phosphorus is present in

to the ability of Ph₂Ppy to act as a proton relay. An additional

possibility is that Ph₂Ppy acts as a hemilabile ligand to stabilize

the catalyst.¹⁷Drent’s initial report has been followed by several

others exploring new P,N-bidentate ligands^18–23and the mechanism

of the catalysis.^{7,17,22,24–27}Elsevier has recently developed variants

of the pyridylphosphine ligand for catalysis in supercritical CO₂

and Doherty has described polymer bound Pd/pyridylphosphine

catalysts.²⁸

The explanation for the enhanced catalytic activity of Ph₂Ppy

vs. PPh₃is still open to debate since a general consensus regarding

the catalytic reaction mechanism has, as yet, not been reached

with both Pd(0)^13–15and Pd(II)^7,17,25intermediates being proposed

in which the pyridylphosphine ligands adopt monodentate,^24–26

or mixed mono- and bi-dentate^7,17coordination, or in which the

coordination mode of the ligand is not deﬁned.²⁷Thus, although

thereappears general agreement thatthe ligandmay act as aproton

relay, the possibility that the hemilability of the ligand is also

important has received little attention, indeed the coordination

chemistry of 2-pyridyldiphenylphosphine with palladium(II) and

the potential for hemilability of Ph₂Ppy at Pd(II) has been little

explored.³

22

2

each complex consistent with the formulations [Pd(k -Ph₂Ppy)Cl₂]

1

(1), cis-[Pd(k -Ph₂Ppy)₂Cl₂] (2a) (d(P) 29.0 (s) ppm), and trans-

[Pd(k -Ph₂Ppy)₂Cl₂] (2b) (d(P) 22.8 (s) ppm).^{34,37,39,42,43}A mixture

1

of cis- and trans-[Pd(k -Ph₂Ppy)₂Cl₂] (2a/b) can be obtained

analytically pure and in high yield as a yellow solid by treatment

of [Pd(PhCN)₂Cl₂], in dichloromethane with 2 equivalents of

Ph₂Ppy, followed by precipitation with diethyl ether as previously

reported by Balch et al.³⁸[Pd(COD)Cl₂] and [Pd(TMEDA)Cl₂]

can also be used as the palladium source in the reaction. Yellow

crystals, suitable for an X-ray crystal structure determination were

obtained by layering a dichloromethane solution of a mixture of

cis and trans isomers 2a/b with diethyl ether (see ESI†). Visually,

there appeared to be only one type of crystal present,⁴⁴which

the X-ray determination showed to contain the trans isomer 2b

(Supplementary Fig. 44, Supplementary Table 3, ESI†). However,

Our interest in Pd/phosphine catalyzed hydrocarboxylation

of alkenes and alkynes has led us to investigate the chemistry

of the potentially hemilabile 2-pyridyldiphenylphosphine ligand

(Ph₂Ppy) at Pd(II). Transition-metal complexes of pyridylphos-

phine ligands have been well known for many years.³Al-

though both monodentate and bridging coordination modes

of pyridylphosphines to Pd(II), and chelating coordination to

other transition metals are well documented, it is surprising

that examples of a pyridylphosphine ligand chelating a Pd(II)

centre are rare.²⁹In palladium(II) chemistry, monodentate com-

plexes are usually formed by the binding of the soft phos-

phorus atom to the metal centre; no N-bound monodentate

complexes have been reported and there are only two pre-

1

the ³¹P{ H} NMR spectrum recorded immediately on dissolution

of the crystals showed a mixture of 2a and 2b. This seems surprising

in view of the sharpness of both resonances, indicating that

interconversion of the isomers is slow on the NMR timescale

at 293 K. The proportions of 2a and 2b present in solution is

temperature dependent. For example in CD₂Cl₂the ratio 2a : 2b

is 70 : 30 at 293 K and 97 : 3 at 193 K. Furthermore, Balch has

reported that it is the cis and not the trans complex that is involved

in the exchange process in the analogous platinum system.³⁷Anion

assisted ﬂuxional processes in these systems are discussed later

and might account for the rapid interconversion of trans and cis

isomers.

Balch et al.

2

45

vious reports of k -pyridyphosphine chelating complexes of

have previously reported the existence of an

2

1

Pd(II) – [Pd(k -Me₂Ppy)(k -Me₂Ppy)X]Y, (X = Cl, Br, I, Y =

equilibrium between cis-[Pt(k -Ph₂Ppy)₂X₂] (X = I, Cl) and

2

[ClO₄], [PF₆])³⁰and the pyrimidylphosphine complex [Pd{k -

2

1

“chelate–monodentate” [Pt(k -Ph₂Ppy)(k -Ph₂Ppy)X](X) (X = I,

Cl) in chloroform solution. The position of the equilibrium shifts

in favour of the cationic “chelate–monodentate” complex at low

temperature, which is attributed to the entropic contribution of

two monodentate Ph₂Ppy ligands vs. one monodentate Ph₂Ppy lig-

and becoming less signiﬁcant at low temperature. Thus, ionization

of halide from the platinum centre in [Pt(Ph₂Ppy)₂X₂] (assisted by

intramolecular attack of a pyridyl nitrogen) in chloroform solution

is feasible. This may reﬂect the greater propensity of square-

planar platinum complexes to react by a dissociative pathway.⁴⁶

By contrast, we see no evidence for the formation of a “chelate–

monodentate” structure from the palladium complexes 2a/b in

dichloromethane at low temperatures.

Ph₂P(C₄H₃N₂)}₂][BF₄]₂.²⁰The X-ray crystal structures of the small

number of chelating four-membered ring complexes containing

Ph₂Ppy (at metals other than Pd(II)), reveal a highly compressed

P–M–N angle of ca. 70^◦.^29–32Within the chelate ring, there is also

signiﬁcant angular compression. Both the C–N–M and the N–

C–P angles are reduced from the ideal value of 120^◦and the

C–P–M angle is reduced below the expected value of 109.5^◦.

Furthermore, the chelate P–C distance is longer than the non-

chelate P–C distances. The ring strain in these chelated complexes

render them unstable, thus the nitrogen donor is readily displaced

from the low-valent metal giving phosphorus bound monodentate

complexes.⁵In many instances binuclear complexes in which the

ligand bridges the metal–metal bond are preferred.^31–40

There are

a few reports of the formation of P,N

2

chelates, including the closely related complexes [Pt(k -

Ph₂Ppy)(k -Ph₂Ppy)Cl][Rh(CO)₂Cl₂],³¹and [Pd(k -Me₂Ppy)(k -

1

2

1

Results and discussion

Me₂Ppy)X](Y), (X = Cl, Br, I, Y = [ClO₄]^-, [PF₆]^-),³⁰and

[Pt(k -Ph₂Ppy)(k -Ph₂Ppy)X][BPh₄]²⁹the latter complexes being

prepared by halide abstraction using silver salts from dihalide

complexes [ML₂X₂]. This approach proved unsuccessful here.

However, we ﬁnd that addition of a few drops of MeOH

to a dichloromethane suspension of 2a/b results in complete

2

1

Synthesis of Pd(II) complexes with both chelating and monodentate

Ph₂Ppy ligands

Direct addition of Ph₂Ppy to [Pd(PhCN)₂Cl₂] in dichloromethane

solution, (metal : ligand ratio varied from 4 : 1 to 1 : 2) gave a

mixture of three complexes each of which shows a sharp singlet in

dissolution of the solid. ³¹P{ H} NMR spectroscopy revealed the

1

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1

Table 1 ³¹P{ H} NMR data for Pd(II)–Ph₂Ppy compounds at 193 K in dichloromethane unless otherwise stated^a

Compound

d(P¹)^b/ppm

d(P²)^b/ppm

d(P³)/ppm

²J(P¹–P²)/Hz

²J(P²–P³)/Hz

2

1

[Pd(k -Ph₂Ppy)Cl₂]

—

-61.6 s

—

-44.0 d

-43.8 d

-43.9 d

-43.8 d

—

-39.3 d

-40.0 d

-38.6 d

-45.0

-45.8 br

-43.7

-42.9 br

-47.7 d br

—

33.0

38.2

38.1

38.2

—

Unresolved

29.8

24.4

—

2a

cis-[Pd(Ph₂Ppy)₂Cl₂]^c

29.0 s

22.8 s

27.6 br

19.1 s

44.4 d

44.8 d

44.6 d

44.7 d

33.1 br

33.2 br

30.2 s

40.8 d

41.2 d

40.8 d

26.4

2b

trans-[Pd(Ph₂Ppy)₂Cl₂]^c

1

2[CF₃CO₂]

2b¢

[Pd(k -Ph₂Ppy)₂(CF₃CO₂)₂]

d

trans-[Pd(Ph₂PpyH)₂Cl₂][MeSO₃]₂

2

1

3(Cl)

[Pd(k -Ph₂Ppy)(k -Ph₂Ppy)Cl](Cl)^de

2

1

3[BF₄]

3[OTf]

3[MeSO₃]

4(Cl)

[Pd(k -Ph₂Ppy)(k -Ph₂Ppy)Cl][BF₄]^de

2

1

[Pd(k -Ph₂Ppy)(k -Ph₂Ppy)Cl][OTf]^de

2

1

[Pd(k -Ph₂Ppy)(k -Ph₂Ppy)Cl][MeSO₃]

—

1

[Pd(k -Ph₂Ppy)₃Cl](Cl) ^ef

‡

25.7 br

25.6 br

—

unresolved

< 8

—

1

4[OTf]

7

[Pd(k -Ph₂Ppy)₃Cl][OTf]^ef‡

1

[Pd(k -Ph₂Ppy)₂(p-benzoquinone)]

2

1

8[OTf]

8[MeSO₃]

8[CF₃CO₂]

8¢[OTf]

8¢[MeSO₃]

8¢[BF₄]

9

[Pd(k -Ph₂Ppy)(k -Ph₂Ppy)(OTf)][OTf]

2

1

[Pd(k -Ph₂Ppy)(k -Ph₂Ppy)(MeSO₃)][MeSO₃]

2

1

[Pd(k -Ph₂Ppy)(k -Ph₂Ppy)(CF₃CO₂)][CF₃CO₂]

2

1

[Pd(k -Ph₂Ppy)(k -Ph₂PpyH)(OTf)][OTf]₂

2

1

[Pd(k -Ph₂Ppy)(k -Ph₂PpyH)(MeSO₃)][MeSO₃]₂

26.0 br

25.2

—

Unresolved

—

2

1

[Pd(k -Ph₂Ppy)(k -Ph₂PpyH)(BF₄)][BF₄]₂

2

[Pd(k -Ph₂Ppy)₂][BF₄]₂

—

23.6 d br

—

388

2

1

g

10

[Pd(k -Ph₂Ppy)(k -Ph₂Ppy)₂][MeSO₃]₂

44.7 s br

Unresolved

^aSee Experimental section for ¹H and ¹⁹F NMR. ^bSee Scheme 1 for numbering scheme. ^cAt 293 K. ^dIn dichloromethane–MeOH solution. ^eAt 233 K.

^fP1 trans to Cl, see Supplementary Fig. 37, ESI†. ^gSee Scheme 4 for numbering scheme.

formation of the cationic Pd(II)“chelate–monodentate” complex

resonances, Table 1 (Supplementary Fig. 2a, ESI†). However, the

dynamic behaviour of the NMR spectra is counterion dependent,

e.g. the VT spectra of 3(Cl) show signiﬁcant broadening as

the temperature is raised to 293 K, whereas those of 3[BF₄]

remain sharp, Supplementary Fig. 2a and b, ESI†. This dynamic

behaviour is discussed later.

2

1

[Pd(k -Ph₂Ppy)(k -Ph₂Ppy)Cl](Cl), 3(Cl) (Scheme 1). Thus, the

1

³¹P{ H} NMR spectrum of 3(Cl) at 233 K shows two sets of

2

doublets at d(P) 44.4 (d) and -44.0 (d) ppm, J(P¹–P²) = 33.0

Hz indicating the presence of two mutually cis, inequivalent

phosphorus atoms, Table 1 (Supplementary Fig. 2a, ESI†). The

resonance at very low frequency can be assigned on the basis of

its chemical shift to the phosphorus of a chelating Ph₂Ppy ligand⁴¹

with P trans to Cl, P²in Scheme 1. The resonance at high frequency

(d(P) 44.4 ppm) is consequently assigned to the phosphorus of

a monodentate pyridylphosphine ligand trans to the chelating

nitrogen of the pyridine ring, P¹in Scheme 1. The breadth of

the resonances at 293 K is attributed to counterion assisted

displacement of the coordinated chloride ligand by nitrogen

(vide infra). The “chelate–monodentate” structure of 3 has been

Crystal structure of [Pd(j²-Ph₂Ppy)(j¹-Ph₂Ppy)Cl][BF₄]

(3[BF₄]). Crystals of 3[BF₄] suitable for an X-ray crystal structure

determination were obtained by layering a dichloromethane

solution of 3[BF₄] with diethyl ether. Fig. 1 shows a representation

of 3[BF₄], selected bond lengths and angles are given in Table 2.

2

conﬁrmed by an X-ray crystal structure determination of [Pd(k -

1

Ph₂Ppy)(k -Ph₂Ppy)Cl][BF₄], 3[BF₄] which can be obtained as an

analytically pure yellow powder by addition of 1 equivalent of

Ag[BF₄] to a dichloromethane–MeOH (8 : 1) solution of 3(Cl),

followed by precipitation with excess ether.

2

1

Fig.

1

Molecular structure of [Pd(k -Ph₂Ppy)(k -Ph₂Ppy)Cl][BF₄]

Scheme 1 Synthetic route to the ionic Pd(II) “chelate–monodentate”

complex 3(Cl).

(3[BF₄]), with H atoms removed for clarity.

The coordination about the palladium is distorted square

planar. One of the pyridylphosphine ligands chelates the Pd

through nitrogen and phosphorus while the other functions as

a monodentate phosphorus bound ligand. The two phosphorus

Salts of 3 with other counterions can be prepared by the addition

of one to four equivalents of TlX to a dichloromethane solution

of 2 or to a dichloromethane–methanol solution of 3(Cl), to give

1

3(X), (X = BF₄, OTf, MeSO₃). The ³¹P{ H} NMR spectra of 3(X)

atoms are mutually cis in agreement with the ³¹P{ H} NMR

in CH₂Cl₂at 233 K are essentially independent of X, showing

two sharp doublets at d(P) 44.7 and -43.8 ( 0.1) ppm with ²J(P¹–

P²) = 38.2 Hz with only small counterion dependent shifts of the

spectrum recorded in solution. The Pd–P bonds have essentially

˚

the same lengths, 2.2644(4) and 2.2330(4) A. The Pd–N bond

˚

length is 2.1075(2) A, in agreement with the closely related cation

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◦

2

1

˚

becoming very broad at 193 K (Supplementary Fig. 32, ESI†). The

¹H NMR spectra of 4(Cl) show similar VT behaviour becoming

very broad at low temperature and are uninformative. At 233 K

Table 2 Selected bond lengths (A) and angles ( ) for [Pd(k -Ph₂Ppy)(k -

Ph₂Ppy)Cl][BF₄] (3[BF₄])

Pd–P1

Pd–N1

2.2644(4)

2.1075(2)

Pd–P2

Pd–Cl1

2.2330(4)

2.3225(5)

1

the ³¹P{ H} NMR spectrum shows two resonances of intensity

1 : 2 at d(P) = 33.1 and 25.7 ppm as expected. Couplings between

the resonances are not resolved, although shoulders are just

discernible on the resonance of 4[OTf] at d(P) = 33.2 ppm at

233 K, consistent with a triplet due to coupling to two equivalent

phosphorus nuclei, ²J(PP) < 8 Hz. At 193 K at least three

resonances can be distinguished at ca. 32.4, 27.4 and 25.2 ppm,

indicating a very dynamic system in which the trans disposed

Ph₂Ppy ligands become inequivalent to each other. These dynamic

processes are discussed later. Attempted precipitation of 4(Cl) by

addition of diethylether afforded an impure mixture of 2a, 2b and

free Ph₂Ppy. However, slow removal of solvent at reduced pressure

afforded crystals of 4(Cl) suitable for an X-ray crystal structure

determination (see ESI†). 4[OTf] can be precipitated in diethyl

ether and redissolved in e.g. CDCl₃.

P2–Pd–P1

P2–Pd–Cl1

P1–C5–N1

N1–C5–C4

C5–N1–C1

C5–N1–Pd

N1–Pd–P2

101.666(14)

90.134(14)

103.38(10)

122.30(13)

120.13(13)

101.73(9)

N1–Pd–Cl1

N1–Pd–P1

P1–C5–C4

N2–C18–P2

Pd–N1–C1

C5–P1–Pd

P1–Pd–Cl1

97.54(3)

70.45(3)

134.24(11)

117.01(11)

136.20(10)

82.63(5)

168.19(3)

167.983(15)

2

1

[Pd(k -Me₂Ppy)(k -Me₂Ppy)Cl]⁺.³⁰A terminal chloride ligand

completes the coordination sphere. The presence of a strained four-

membered chelate ring results in the interbond angles at palladium

deviating strongly from the ideal value of 90^◦.

Thus, P1–Pd–N1 is only 70.45^◦. This compression causes the

two adjacent angles to open up, N1–Pd–Cl is 97.54^◦and P1–Pd–

P2 is 101.66^◦. The P2–Pd–Cl angle is 90.13^◦, close to the ideal

value. There is also signiﬁcant angular compression within the

chelate ring. Thus, C5–N1–Pd and P1–C5–N1 are 101.73 and

103.38^◦, respectively, reduced from an ideal 120^◦and the C5–P1–

Pd angle is 82.63^◦, reduced from the expected value of 109.5^◦.

The angular compression seen in this ring is very similar to that

Other strongly coordinating ligands such as Cl^-and PPh₃also

disrupt the four-membered chelate in 3(Cl) although the resultant

1

³¹P{ H} NMR spectra show a mixture of unidentiﬁable products.

However, other potential donors relevant to alkyne hydroesteriﬁ-

cation catalytic systems appear not to disrupt the chelate structure

readily. Thus, bubbling CO through a dichloromethane–MeOH

solution of 3(Cl) at 293 K and ambient pressure does not result

in displacement of the chelating nitrogen. Liu et al. has previously

commented on the relatively low afﬁnity of CO for Pd(II) centres in

found in the chelate ring of [Pt(k -Ph₂Ppy)(k -Ph₂Ppy)Cl]⁺.³¹In

addition, there is a weak donor acceptor interaction between

the phosphorus centre of the chelating ligand and the nitrogen

atom of the monodentate phosphine. The distance P1 ◊ ◊ ◊ N2 is

2.7472(14) A, which is well below the sum of their van der Waals

radii of 3.35 A. The P1 ◊ ◊ ◊ N2 interaction is accompanied by

2

1

these systems.^50,51Neither is the nitrogen displaced by MeC CH

≡

or PhC CH.

˚

47

˚

Preparation of “chelate–monodentate” complexes by halide ab-

straction. The presence of a tightly bound halide ligand in the

fourth coordination site at Pd(II) has been observed to suppress

strongly the activity of the Pd centre in organometallic catalytic

reactions. Thus, for example, van Leeuwen et al. reported that the

rate of carbonylation of the Pd–Me bond in several palladium–

diphosphine complexes [(P–P)PdMe(solv)]⁺, (P–P = diphosphine

ligand, solv = solvent) is at least 10 times higher than those of the

corresponding neutral chloride complexes,⁵²and Liu et al.^50,51,53

have recently shown that the presence of a strongly coordinating

ligand such as chloride at the metal centre can have a profound

effect on the individual reaction steps in the hydroalkoxycar-

bonylation of alkenes catalysed by Pd(II) diphosphine cations,

in unfavourable cases completely inhibiting the reaction. We

therefore attempted to remove the coordinated chloride from 3(X)

and replace it with a labile ligand. As noted above, coordinated

chloride is not removed on addition of excess Tl(X). Addition

of an excess of Ag[BF₄], to a dichloromethane–MeOH solution

(8 : 1) of either 3(Cl) or 3[BF₄] gave several unidentiﬁed products,

however, formation of Pd black was not observed. Addition of

MeCN to the reaction solution resulted in an immediate colour

change from orange to yellow and the formation of a new complex

5. We have previously shown that the addition of a moderately

coordinating ligand such as MeCN can stabilise the Pd(II) metal

a wider Ph–P–Ph angle [C(Ph)–P1–C(Ph) 111.28(7) and C(Ph)–

˚

P2–C(Ph) 108.01(7) A]. This interligand interaction appears to

optimise the ligand packing within the square planar coordination

sphere. A search of the Cambridge Structural Database revealed

that short interligand P ◊ ◊ ◊ N contacts between dangling pyridines

and metal-coordinating phosphines are not uncommon but have

not previously been noted in the relevant literature. For example,

2

1

the corresponding Pt-complex [Pt(k -Ph₂Ppy)(k -Ph₂Ppy)Cl]⁺ex-

˚

hibits an even shorter interligand P ◊ ◊ ◊ N interaction of 2.67 A

(in conjunction with a signiﬁcantly wider Ph–P–Ph angle of

115.4^◦at the N-interacting phosphorus centre).³¹The P ◊ ◊ ◊ N

interaction between dppm and one pyridine-2-thiolate ligand

2

1

in [Pt(k -dppm)(k -py-2-S)₂] is similarly short.⁴⁸Incidentally, the

latter features the largest Ph–P–Ph angle (114.5^◦) of all structurally

characterized square planar Pd- and Pt-dppm complexes (mean

106.5^◦).

Reaction of 3 with donor substrates. The nitrogen of the chelate

ring can be displaced in methanol–dichloromethane solution

1

by addition of another equivalent of Ph₂Ppy to give [Pd(k -

Ph₂Ppy)₃Cl](Cl) (4(Cl)) which can be converted to the triﬂate

1

salt [Pd(k -Ph₂Ppy)₃Cl][OTf] (4[OTf]) by reaction with Tl[OTf].

Reaction with Ag[BF₄] affords a complex mixture which includes

silver–Ph₂Ppy complexes;⁴⁹this mixture has not been further

characterized. The formation of silver complexes presumably

1

centre in analogous systems.^50,51The ³¹P{ H} NMR spectrum of

1

reﬂects the labile nature of the Ph₂Ppy ligands in 4. The ³¹P{ H}

5 in CH₂Cl₂–MeCN (20 : 1) solution at 193 K shows two doublets

at d(P) 41.3 and -46.3 ppm, ²J(P¹–P²) = 38.3 Hz indicative of the

presence of a “chelate–monodentate” structure (Supplementary

Fig. 4, ESI†). However, we have been unable to isolate 5 and

NMR spectrum of 4(Cl) shows a single, very broad resonance

around 26 ppm at 293 K. On cooling the solution, the resonances

sharpen between 273 and 223 K then broaden again below 223 K

7924 | Dalton Trans., 2010, 39, 7921–7935

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2

the ¹H NMR spectrum is uninformative due to exchange of

The p-benzoquinone molecule is k -coordinated as a mono-

1

solvent and coordinated MeCN. 5 is tentatively suggested to be

oleﬁnic ligand ensuring a cis orientation of the k -Ph₂Ppy ligands

about the metal centre. The coordinated double bond of the p-

benzoquinone shows signiﬁcant lengthening compared with the

uncoordinated double bond [1.426(9) vs. 1.320(9) A] (C(sp2)–

C(sp2) single bond range is 1.45 to 1.51 A whereas the C C

double bond of free H₂C CH₂is 1.337 A) and there is a

2

1

[Pd(k -Ph₂Ppy)(k -Ph₂Ppy)(MeCN)][BF₄]₂. Therefore, we turned

our attention to other routes to halide-free chelate–monodentate

complexes.

˚

=

55

=

Halide-free routes to Pd(II)–Ph₂Ppy complexes

slight twist (14.3^◦) in the PdP₂to PdC₂planes, showing this

is more appropriately described as a metallacyclopropane. The

twist is slightly larger (14.3 vs. 9.3^◦) than in the closely related

complex [Pd(d^tbpx)(p-benzoquinone)],⁵⁴which may reﬂect the

increased electron density at the palladium centre in the d^tbpx

complex favouring metallacyclopropane formation and/or the

1

From Pd(0) precursors. Dervisi has reported that [Pd(k -

Ph₂Ppy)₂(dba)] (6) is obtained on reaction of [Pd₂(dba)₃] with

Ph₂Ppy.²⁴Similarly, we ﬁnd 6 rather than the desired “chelate–

monodentate” is obtained in MeOH. Zacchini showed that on

treatment with benzoquinone, in the absence of acid, dba is

replaced by p-benzoquinone in [Pd(d^tbpx)(dba)] (d^tbpx = 1,2-

(CH₂PBu^t₂)₂C₆H₄),⁵⁴in THF solution, affording [Pd(d^tbpx)(p-

steric constraint of the k -coordinated d^tpbx ligand compared

2

to the less bulky Ph₂Ppy monodentate ligands of 7. The ligand

1

benzoquinone)]. We now report that treatment of [Pd(k –

has a dihedral angle of 94.28^◦to the PdC₂coordination triangle,

Ph₂Ppy)₂(dba)] with 1 equivalent of p-benzoquinone affords the

2

in keeping with the k -coordination of the alkene. The steric

hindrance caused by the phenyl rings forces the benzoquinone

1

new complex [Pd(k -Ph₂Ppy)₂(p-benzoquinone)] (7) in high yield

as an analytically pure red solid. 7 has been characterized by an

X-ray crystal structure determination. In contrast to 6, 7 provides

a convenient, halide-free route to Pd “chelate–monodentate”

complexes of Ph₂Ppy.

=

ligand to “bow”, the two C O bonds are inclined away from the

metal centre, opening up the P–Pd–C angles (average 146^◦).

1

Reactions of [Pd(k -Ph₂Ppy)₂(p-benzoquinone)] (7). Addi-

tion of 2 equivalents of MeSO₃H to 7 results in an immedi-

1

ate colour change from yellow to orange. The ³¹P{ H} NMR

1

Crystal structure of [Pd(k -Ph₂Ppy)₂(p-benzoquinone)] (7).

Crystals of 7, suitable for an X-ray crystal structure determination,

were obtained by layering a dichloromethane solution of 7 with

petroleum ether. The molecular structure of 7 is shown in Fig. 2

and selected bond lengths and angles are given in Table 3. The

coordination about the metal centre is distorted square planar

(if the p-benzoquinone ligand is considered to occupy two sites).

Both of the Ph₂Ppy ligands are coordinated to palladium through

spectrum reveals the presence of a new complex proposed

2

1

to be [Pd(k -Ph₂Ppy)(k -Ph₂Ppy)(MeSO₃)][MeSO₃] (8[MeSO₃]).

8[MeSO₃] shows resonances at d(P) = 41.2, and -40.0 ppm, ²J(P¹–

P²) = 29.8 Hz, indicative of the formation of a Pd(II) “chelate–

monodentate” structure. Attempts to precipitate this complex

as a solid were unsuccessful, impure oils being obtained (vide

infra for further characterization of complexes of type 8). If

˚

2

phosphorus only. The average Pd–P bond lengths, 2.307 A, are

excess acid is used in the synthesis, a different complex [Pd(k -

Ph₂Ppy)(k -Ph₂PpyH)(MeSO₃)][MeSO₃]₂(8¢[MeSO₃]₂), which is

1

comparable to average bond lengths in the closely related complex

[Pd(PPh₃)₂(p-benzoquinone)].⁵⁵

formed from 8[MeSO₃], is obtained. Thus, addition of 1 equivalent

of MeSO₃H to a dichloromethane solution of 8[MeSO₃] under

ambient conditions gives a mixture, 8[MeSO₃] and 8¢[MeSO₃]₂.

8¢[MeSO₃]₂shows two broad resonances, at d(P) = 26.0 and

-45.8 ppm in its ³¹P{ H} NMR spectrum, Table 1. The former

1

resonance is in the region expected for monodentate PPh₂py,

while the latter resonance shows the characteristic low frequency

shift for a chelating PPh₂py ligand. The ³¹P NMR resonances

of 8¢[MeSO₃]₂integrate 1 : 1 consistent with 8¢[MeSO₃] being a

Pd ‘chelate–monodentate’ species. Both resonances show a low

frequency shift compared to 8, that of the monodentate PPh₂py

ligand being particularly marked, consistent with protonation of

the pyridine ring of the monodentate ligand.⁶Addition of further

aliquots of MeSO₃H results in complete conversion of 8[MeSO₃]

1

Fig. 2 The molecular structure of [Pd(k -Ph₂Ppy)₂(p-benzoquinone)] (7).

H atoms removed for clarity.

1

to 8¢[MeSO₃]₂. Complexes with essentially identical ³¹P{ H} NMR

spectroscopic data can be obtained by addition of 2 equivalents

of Ph₂Ppy and 3 equivalents of acid to solutions of [Pd(OAc)₂]

(Supplementary Fig. 6a, ESI†, vide infra). The ¹H NMR spectrum

of 8¢[MeSO₃]₂at 193 K shows a broad resonance at ca. d(H) =

17.1 ppm that integrates 1 : 1 against the H⁶proton of each

pyridyl ring and is assigned to protonation at nitrogen.³⁴The

protons of one of the pyridyl rings also show high frequency

shifts consistent with protonation of the nitrogen, for example the

H⁶proton shifts from d(H) = 9.17 to 9.48 ppm (Supplementary

Fig. 5b and 6b, ESI†).⁶Scrivanti has also recently studied the

protonation of pendant pyridylphosphine ligands at Pd(0) and

Pd(II).⁶We therefore decided to investigate further whether this

◦

1

˚

Table 3 Selected bond lengths (A) and angles ( ) for [Pd(k -Ph₂Ppy)₂(p-

benzoquinone)] (7)

Pd–P1

2.316(2)

2.141(6)

1.426(9)

1.228(8)

Pd–P2

2.299(2)

2.187(5)

1.320(9)

1.233(7)

Pd–C18

C18–C23

C19–O2

Pd–C23

C20–C21

C22–O1

C18–Pd–C23

P1–Pd–C18

38.5(2)

149.1(2)

P1–Pd–P2

P2–Pd–C23

105.56(5)

142.4(2)

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1

above, the low-frequency shift of the ³¹P{ H} NMR resonance of

new complex, 8¢[MeSO₃]₂, was indeed 8[MeSO₃] protonated at the

pendant pyridyl nitrogen. Addition of 1 equivalent of MeSO₃H

to a CH₂Cl₂solution of Ph₂Ppy results in protonation at nitrogen

but not phosphorus to give [Ph₂PpyH][MeSO₃]. Thus, the ¹⁵N

resonance shifts to lower frequency from d(N) = 51.2 ppm (²J(N–

H) = 11 Hz and ²J(N–P) = 34 Hz) to d(N) -168.5 ppm (²J(N–H) =

80 Hz and ²J(N–P) = 18 Hz) (Supplementary Fig. 8, ESI†). The

the monodentate Ph₂Ppy ligand, and the appearance of a broad

resonance around 17 ppm in the H NMR spectra of complexes

1

8¢ are consistent with protonation of this ligand. Addition of

intermediate equivalents of acid to complexes 8 afford mixtures of

8 and 8¢, again consistent with the proposed formulations. For X =

BF₄a different complex is obtained on addition of two equivalents

of HBF₄. This complex shows a single broad peak at -42.9 ppm in

1

³¹P{ H} NMR spectrum of [PPh₂pyH][MeSO₃], in CH₂Cl₂at 298

the ³¹P{ H} NMR spectrum consistent with a chelating structure.

1

K, shows a sharp singlet at d(P) -7.8 ppm, shifted to low frequency,

relative to the free ligand (d(P) -4.5 ppm). No additional coupling

Addition of further equivalents of HBF₄converts this complex

into 8¢[BF₄]₂.

on the phosphorus resonance is observed in the proton coupled ³¹

P

NMR spectrum, indicating protonation on the phosphorus atom

has not occurred.

We have only been able to obtain impure oils or solids of 8 and 8¢

and the ¹H and ¹⁹F NMR spectra of 8[X] and 8¢[X]₂are uninforma-

tive regarding the ligand in the fourth coordination site at Pd due to

exchange of coordinated X and the counterion, (X = OTf, MeSO₃,

BF₄). However, the spectra of compounds containing the more

coordinating triﬂuoroacetate anion are informative. Addition of

CF₃COOH to a CH₂Cl₂solution of [Pd(OAc)₂] as described above,

affords a solution containing the proposed chelate–monodentate

complex 8[CF₃CO₂]. Precipitation with Et₂O affords a solid

analyzing for “Pd(Ph₂Ppy)₂(CF₃CO₂)₂”. On redissolution of the

Protonation of the pendant pyridyl nitrogen is proposed to be an

important step in the catalytic hydroesteriﬁcation of alkynes using

Pd/pyridylphosphine catalysts. Thus, both Drent¹⁷and Scrivanti²⁷

report that an excess of strong acid is required to achieve high

activities in the carbonylation of alkynes catalysed by Pd/PPh₂py

complexes and proposed that the excess acid is required to

protonate the nitrogen of a pendant pyridylphosphine ligand.

Subsequent transfer of this proton to the substrate occurs either

in the termination, Scheme 2, or initiation step, Scheme 3 and

is proposed to account for the high activity of pyridylphosphine,

compared to triarylphosphine complexes, in this catalytic system.

1

solid in CH₂Cl₂, the ³¹P{ H} NMR spectrum at 193 K (Fig. 3a)

reveals the presence of two complexes, in the ratio 7 : 1, the major

complex showing a broad singlet around d(P) = 27 and the minor a

pair of broad resonances at 40.8 and - 38.6 ppm. This is consistent

with the major compound being a bis-monodentate complex

1

analogous to 2, [Pd(k -Ph₂Ppy)₂(CF₃CO₂)₂] ([2(CF₃CO₂)₂]) and

the minor being the chelate–monodentate complex 8[CF₃CO₂].

The ¹⁹F NMR spectrum (Fig. 3c) shows a sharp singlet at d(F) =

-75.9 consistent with the presence of a CF₃COO₂^-counterions and

a broad asymmetrical resonance which deconvolutes to minor and

major resonances at d(F) = -74.0 and -74.5, respectively, shifts

-

consistent with coordination of CF₃COO₂to Pd, i.e. consistent

with the presence of [2(CF₃CO₂)₂] and 8[CF₃CO₂], eqn (1), (cf .

[Et₃NH][CF₃CO₂] d(F) = -76.1, [Pd(OAc)₂] + CF₃CO₂H d(F) =

-74.6).

Scheme 2 Pyridylphosphine as a ‘proton messenger’ in the termination

step of the methoxycarboxylation of alkynes, as proposed by Drent.

+MeOH

1

ꢀꢀꢀꢀꢀꢁ

ꢂꢀꢀꢀꢀꢀ

[Pd(k -PPh py) (CF CO ) ]

2

3

2 2

(1)

[Pd(k¹-PPh₂py)(k²-PPh₂py)(CF CO₂)]+[(CF³CO₂)]⁻

3

Deconvolution of the ¹⁹F NMR spectrum affords a ratio of

the major complex (2) : counterion of 7.5 : 1 consistent with the

Scheme 3 Pyridylphosphine as a ‘proton messenger’ in the initiation step

of the methoxycarboxylation of alkynes, as proposed by Scrivanti.

1

³¹P{ H} NMR spectroscopic data. On addition of methanol

to solvate any liberated CF₃CO₂ions released into solution,

1

changes are observed in the ³¹P{ H}, ¹⁹F, and the pyridyl region

Preparation of chelate–monodentate complexes from Pd(II) pre-

cursors. In an attempt to obtain analytically pure complexes

we have investigated the synthesis of complexes of type 8 from

[Pd(OAc)₂]. Addition of two equivalents of HX, (X = OTf,

MeSO₃) to a dichloromethane solution of [Pd(OAc)₂] and two

of the ¹H NMR spectra (Fig. 3, Supplementary Fig. 5b and

6b, ESI†) consistent with a change in the predominant coor-

dination mode of the Ph₂Ppy ligands from bis-monodentate to

chelate–monodentate. The ratio of [2(CF₃CO₂)₂] to 8[CF₃CO₂]

1

determined by integration of the ³¹P{ H} NMR spectrum and

2

1

equivalents of PPh₂py affords [Pd(k -Ph₂Ppy)(k -Ph₂Ppy)(X)][X]

(8[X]), in which one anion X is coordinated to palladium and

one anion X is present as a counterion. Addition of further

by deconvolution of the ¹⁹F NMR spectrum in the presence of

MeOH is 1 : 7 and 1 : 10, respectively, values in good agreement

with eqn (1) within the error limits of the measurements. Taken

together, these data are strongly indicative that the formulations

of [2(CF₃CO₂)₂] and 8[CF₃CO₂], and hence of 8 more generally, as

2

1

equivalents of HX, affords [Pd(k -Ph₂Ppy)(k -Ph₂PpyH)(X)][X]

1

(8¢[X]₂). The ³¹P{ H} NMR spectra of 8[X] show slight variation

2

1

with X, as do those of 8¢[X]₂and are consistent with the formula-

[Pd(k -Ph₂Ppy)(k -Ph₂Ppy)(X)][X] are correct. The formulation of

2

1

2

1

tions [Pd(k -Ph₂Ppy)(k -Ph₂Ppy)(X)][X] and [Pd(k -Ph₂Ppy)(k -

8¢ as the protonated form of 8 is established by the ¹H and ³¹P{ H}

Ph₂PpyH)(X)][X]₂(Supplementary Fig. 6a, ESI†). As discussed

NMR spectra discussed above.

7926 | Dalton Trans., 2010, 39, 7921–7935

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◦

1

˚

Table 4 Selected bond lengths (A) and angles ( ) for trans-[Pd(k -

Ph₂PpyH)₂Cl₂][MeSO₃]₂(2b¢)

Pd–P1

Pd–P2

2.301(2)

2.335(2)

Pd–Cl1

Pd–Cl2

2.287(2)

2.297(2)

P1–Pd–P2

Cl1–Pd–Cl2

P1–Pd–Cl1

176.78(6)

175.82(5)

83.83(6)

P1–Pd–Cl2

P2–Pd–Cl1

P2–Pd–Cl2

92.75(5)

93.07(6)

90.31(5)

phosphorus and are mutually trans, two terminal chloride ligands

complete the coordination sphere. Both pendant nitrogen atoms

of the Ph₂Ppy ligands are protonated. The average M–P bond

˚

lengths (2.317 A) are similar to those found in 2b. The P1–Pd–P2

and Cl1–Pd–Cl2 bond angles.

We noted above that the protonated, chelate–monodentate

complex 8¢[MeSO₃]₂can be prepared, essentially quantitatively,

by addition of excess MeSO₃H to a dichloromethane solution

of [Pd(OAc)₂] and PPh₂py. However, in methanol, a different

complex is formed. Thus, addition of a few drops of MeSO₃H to a

suspension of [Pd(OAc)₂] and 3.1 equivalents Ph₂Ppy in methanol

results in immediate dissolution of the solids to give a red solution.

The major species present in this solution is the previously

2

1

unreported complex [Pd(k -Ph₂Ppy)(k -Ph₂Ppy)₂][MeSO₃]₂(10).

Solutions of 10 are unstable giving unidentiﬁed products on

standing for a few days at 293 K. On evaporation of the solvent, 10

is obtained as an impure red oil. 10 has, therefore, been formulated

Fig. 3 NMR spectra of “Pd(Ph₂Ppy)₂(CF₃CO₂)₂” showing redistribution

1

of the equilibrium of eqn (1) on addition of MeOH. ³¹P{ H} NMR

spectrum in (a) CH₂Cl₂and (b) after addition of MeOH. ¹⁹F NMR

spectrum in (c) CH₂Cl₂and (d) after addition of MeOH.

1

on the basis of its ³¹P{ H} NMR spectrum, Fig. 5. The ¹H NMR

spectra are uninformative, serving only to conﬁrm the presence

1

of the Ph₂Ppy ligand. At 183 K, the ³¹P{ H} NMR spectrum

Although attempts to isolate 8¢ for further characterization were

unsuccessful, we were successful in isolating, and characterizing,

the protonated form of 2b. Thus, addition of 2 equivalents of

MeSO₃H to a dichloromethane solution of 2a/b, results in a new

of 10 shows three sets of broad resonances at d(P) 44.7 ppm

2

(singlet), 23.6 (doublet) and -47.7 (doublet) ppm, J(P²–P³) =

388 Hz, consistent with the presence of two monodentate and

one chelating Ph₂Ppy ligand respectively in the complex. It is not

clear from the chemical shifts of the monodentate ligands, d(P)

44.7 and 23.6 ppm if these ligands are protonated at the pyridyl

nitrogen. The low frequency shift of the latter is consistent with

such protonation but may also reﬂect the differing trans ligands.

In the presence of excess acid, the excess of free PPh₂py ligand is

protonated at the pyridyl nitrogen, resulting in a ca. 1.4 ppm low

frequency shift of the ³¹P NMR resonance to d(P) -8.76 ppm.

1

complex, 2b¢, which shows a sharp singlet in the ³¹P{ H} NMR

spectrum at d(P) = 19.1 ppm. The X-ray crystal structure of 2b¢

conﬁrms protonation occurs at the pyridyl nitrogen.

1

Crystal structure of trans-[Pd(k -Ph₂PpyH)₂Cl₂][MeSO₃]₂

(2b¢). Yellow crystals suitable for an X-ray crystal structure

determination of 2b¢ were obtained by layering a dichloromethane

solution of 2b¢ with diethyl ether. The molecular structure of 2b¢

is shown in Fig. 4 and selected bond lengths and angles are given

in Table 4. The coordination about the palladium centre is square

planar. The Ph₂Ppy ligands are coordinated to palladium through

Dynamic process in chelate–monodentate Pd(II) complexes of

PPh₂py

1

The variable-temperature ³¹P{ H} NMR spectra of 3, 4, 5, 8¢

and 10 reveal that these highly strained chelate–monodentate

complexes are ﬂuxional and we were interested to discover if

this could be due to hemilability of the chelating PN ligand.

Although there is evidence for dissociative processes in Pt(II)

complexes,^56–59dissociative ligand substitution at Pd(II) is ex-

tremely uncommon,^46,60,61and in the presence of an incoming

ligand or solvent, the alternative associative substitution is

much faster. Rare examples of dissociative processes in Pd(II)

2

complexes have been reported by Espinet.^60,61Although [Pd(k -

1

Ph₂Ppy)(k -Ph₂Ppy)₂][MeSO₃]₂might seem an unlikely prospect

for a dissociative process – it is dicationic and does not contain

any electron donating organic ligands that might increase the

electron density at palladium and hence enhance a dissociative

pathway, the presence of the highly strained four-membered

1

Fig. 4 Molecular structure of trans-[Pd(k -Ph₂PpyH)₂Cl₂][MeSO₃]₂(2b¢),

with key atoms labelled.

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Scheme 4 Dynamic processes occurring in 10.

higher temperature processes equivalences P¹N with P²N and P³N

(Scheme 4, eqn (2)). It is not necessary that the higher energy

process simultaneously equivalences all three pyridylphosphine

ligands, only that it equivalences P¹N with one of P²N or P³N

since the lower energy process already equivalences P²N and P³N.

Since this more complex ﬂuxional system should contain more

information about the nature of the ﬂuxional processes occurring,

we began our investigations of these dynamic processes with 10.

Dynamic process in [Pd(j²-Ph₂Ppy)(j¹-Ph₂Ppy)₂][MeSO₃]₂(10).

Due to its instability, solutions of 10 were prepared “in situ” with

nominal Pd : P ratios of 1 : 3 and 1 : 3.4 and nominal Pd : MeSO₃H

ratios of 1 : 2, 10, 15 or 25. This allowed the effects of excess ligand

and excess acid on the ﬂuxional processes to be investigated. We

have also attempted to study the dynamic processes in CD₂Cl₂,

a non-coordinating solvent, and in mixtures of MeOH–CD₂Cl₂.

1

Fig. 5 shows the variable-temperature ³¹P{ H} NMR spectra of

10 in the presence of a slight excess of PPh₂py (Table 5, sample

3). Two coalescence temperatures can be seen clearly, the ﬁrst

occurs around 223 K (Fig. 5d–f) and the second around 263 K

(Fig. 5h and i). Impurities can be seen in many of the spectra

(see e.g. Supplementary Fig. 9, et seq., ESI†) of these in situ

prepared samples. However, it should be noted that equilibrium

saturation transfer difference measurements on impure 10, in

which the resonance of the chelating PPh₂py ligand at ca. d(P)

-48 ppm was irradiated throughout the recovery delay (3 s),

revealed signiﬁcant saturation transfer to the other PPh₂py ligands

in 10 but not to those of other complexes (Supplementary Fig.

27, ESI†) indicating that the impurities do not participate in the

ﬂuxional processes. A small amount of transfer to free [PPh₂pyH]⁺

was seen, indicating exchange with free ligand occurs, albeit

slowly compared to the intramolecular exchange processes. Con-

sistent with intermolecular exchange being slow, the resonance of

[PPh₂pyH]⁺remains sharp until after the onset of both ﬂuxional

processes, Fig. 5 and Supplementary Fig. 19, ESI†, even when

present at low concentration (Supplementary Fig. 13, 17, 21 and

23, ESI†), indicating that free [PPh₂pyH]⁺is not involved in

1

Fig. 5 ³¹P{ H} NMR spectrum of 10 in MeOH: (a) 183 K, (b) 193 K, (c)

203 K, (d) 213 K, (e) 223 K, (f) 233 K, (g) 243 K, (h) 253 K, (i) 263 K, (j)

273 K, (k) 283 K, (l) 293 K. The resonance at ca. d(P) = -8.7 ppm is due

to free [PPh₂pyH][MeSO₃].

1

ring might, nevertheless, confer hemilability on the chelating

Ph₂Ppy ligand. The ﬂuxionality observed in 3, 5, and 8¢ appears

similar – a broadening of the resonances of the phosphorus atoms

of both the chelating and monodentate ligands is observed on

increasing the temperature, although the extent of broadening

depends on the particular complex and counterion present.

The ﬂuxionality observed for 4, and 10 is more complex. For

example, the NMR spectra of 10 (Fig. 5) reveal that two, stere-

ospeciﬁc ﬂuxional processes are occurring. The low-temperature

ﬂuxional process equivalences the chelating and monodentate

2-pyridyldiphenylphosphine ligands P²N and P³N (Scheme 4,

eqn (1)). Such exchange has been brieﬂy described before for

these dynamic processes. The variable-temperature ³¹P{ H} NMR

spectra have been simulated using gNMR5.1 and the activation

parameters obtained, Table 5, which also gives the reaction rate

constants at 213 and 243 K for the low- and high-temperature

processes respectively. The spectra, spectral simulations, Eyring

and Arrhenius plots are given in ESI† (Supplementary Fig. 9–26).

The reaction rate constants of the low- and high-temperature

processes differ by an order of magnitude, conﬁrming that two

independent processes occur. Activation entropies in all samples

are negative, consistent with an associative process, however,

caution must be exercised when interpreting the data since e.g.

changes in solvation might also contribute to the activation

entropy.^46,60,62However, such effects are likely to be minimized in

10 since the potential dissociating ligand, the pyridyl group, is not

2

[Pd(k -Me₂Ppy)(Me₂Ppy)X](X), (X = Cl, Br, I), however the

detailed mechanism of the exchange was not determined.³⁰The

7928 | Dalton Trans., 2010, 39, 7921–7935

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Table 5 Activation parameters (DS^‡/J mol^-1K^-1, DH₁^‡/kJ mol^-1, E_act/kJ mol^-1) for the dynamic processes in 10^a

213

243

‡

c

‡

c

‡

d

‡

d

1

d

2 d

Sample Solvent

Pd/P Pd : MeSO₃H 10^-4k₁^b/s^-110^-4k₂^b/ s^-1DS₁

DS₂

DH₁

DH₂

E_act

1

2

3

4

5

6

7

8

9

MeOH

MeOH–CD₂Cl₂(2 : 9)

MeOH–CD₂Cl₂(4 : 3)

CD₂Cl₂

3.1

3.4

3.2

2.9

3.1

2.9

10

25

15

25

5

1.52

1.23

1.36

1.07^e

1.43

1.11

8.92

4.11

1.15^e

1.91

1.37

1.29

1.90

1.43

1.16

1.69

2.13

1.92^f

-15

-12

-10

-25

-17

-41

-20

-4

-11

-12

-8

-33

-25

-32

-25

-21

31

32

33

29

31

22

24

33

37

38

39

32

34

32

31

35

33

34

35

31

34

25

28

35

39

40

41

34

31

34

33

37

2

^aPrepared in situ. ^bError ~ 10%. ^cError ~ 5 J mol^-1K^-1. ^dError ~ 5%. ^eT = 208 K. ^fT = 238 K.

charged. Changes in solvation are therefore expected to make only

a small contribution to the activation process/parameters unless

protonation–deprotonation of the pyridyl nitrogen contributes

signiﬁcantly.

However, the rate of the low energy process is strongly inﬂuenced

by solvent, being faster in MeOH–CD₂Cl₂mixtures than in pure

MeOH. Inspection of Table 5 reveals that this rate acceleration

is due to a lower enthalpy of activation, being ca. 23 and 33 kJ

mol^-1, respectively (the entropy of activation is more negative in

the mixed solvent). The rate of the dynamic process is marginally

inﬂuenced by the presence of excess acid, compare samples 3 and

4, or with excess ligand, compare samples 1–3.

The stereoselectivity of the low-energy process is easily ex-

Scheme 6 Possible intramolecular high-temperature associative mecha-

nism in 10 involving pseudo rotations and following Hall.⁵⁶

plained by an associative process, Scheme 5. It is reasonable to as-

2

sume that the highly strained Pd(k -Ph₂Ppy) four-membered ring

will prefer to adopt an axial–equatorial, rather than an equatorial–

equatorial, conformation, in any intermediate/transition state of

an associative process. Only two trigonal-bipyramidal intermedi-

ates are then accessible, since the law of microscopic reversibility

requires that the intermediate be symmetrical (A and B in Schemes

5 and 6). A is directly accessible from the square-planar structure

formed on chelation of the nitrogen of P³N. Movement of this

nitrogen into the square plane of the complex displaces the pyridyl

nitrogen of P²N, i.e. stereospeciﬁc equivalencing of P²N and P³N

in 10 can be accomplished without movement of the phosphorus

atoms following the least energetic pathway of Hall^56,63,64and

without involving P¹N, which remains distinct. The observed

solvent effect on the rate of this process might be explained

by competition between methanol solvent molecules and the

incoming pyridyl nitrogen for the apical coordination site.⁶⁰

Structure B, Scheme 6, which is required to equivalence P¹N

by an analogous process, is not directly accessible following the

least energetic pathway proposed by Hall⁵⁶but might be obtained

through a series of pseudo rotations, cf . the Berry pseudo rotation,

Scheme 6, and will presumably, therefore, be subject to a higher

activation barrier. We noted above that the rate of this process is

little affected by changes in the coordinating ability of the solvent,

consistent with a mechanism in which the rate determining step

occurs later in the reaction coordinate. Alternative associative

pathways that could account for the equivalencing of all three

PPh₂py ligands in the higher energy process include a square-

planar to tetrahedral reorganization of the metal centre and

coordination of a fourth PPh₂py ligand to the metal centre dis-

placing the chelating pyridyl nitrogen. The difference in activation

enthalpies of the high- and low-temperature processes is rather

small, ca. 5 kJ mol^-1, we therefore discount the former possibility,

while the marginal effect of excess ligand on the rate would argue

against the latter. Furthermore, the observed broadening of the

1

³¹P{ H} NMR resonance due to PPh₂py, which is present in slight

excess, is small, contrary to the differential broadening that might

be expected. Thus, although the data do not exclude coordination

of a fourth PPh₂py ligand as the pathway of the higher temperature

process, we favour the intramolecular process shown in Scheme 6.

Finally, the stereospeciﬁcities observed could also be explained

by a dissociative process giving a T-shaped “three-coordinate”

solvated intermediate, which might occur in 10 due to the presence

2

of the highly strained Pd(k -Ph₂Ppy) ring. Equivalencing of the

trans disposed ligands, P²N and P³N, is then readily envisioned.

However, Y-shaped structures, which have previously been pro-

posed as high energy transition states, are necessary to interconvert

Scheme 5 Postulated intramolecular low-temperature associative mech-

anism in 10 following that of Hall.⁵⁶

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these ligands with P¹N.⁶⁵Although, there is a recent report of T-

shaped Pd(II) complexes incorporating extremely bulky phosphine

ligands in which the vacant fourth coordination site is stabilized

by an agostic interaction,⁶⁶a dissociative pathway is not consistent

with the negative entropy of activation and low activation barriers

observed.

Equivalencing of the PPh₂py ligands might be achieved by a cis–

trans isomerization at Pd in the transition state/intermediate,

Scheme 8, or via a sequence of Berry pseudo-rotations, cf .

Scheme 9.

Dynamic process in [Pd(j²-Ph₂Ppy)(j¹-Ph₂Ppy)Cl]X (3(X)).

1

The ³¹P{ H} NMR chemical shifts and coupling constant in

3(Cl), 3[OTf], and 3[BF₄] are essentially the same, as is expected

1

since only the counterion has changed.^29,30However, the ³¹P{ H}

NMR spectrum of 3[BF₄] is sharp over the temperature range

298–193 K whilst that of 3(Cl) is signiﬁcantly broadened at 293

K with the spectra of 3[OTf] showing intermediate behaviour

(Supplementary Fig. 2 and 28–31, ESI†), indicating the onset of

a ﬂuxional process that is strongly inﬂuenced by the counterion.

As expected for a second-order process, the rate of the ﬂuxional

process is, qualitatively, found to be concentration dependent.

Analysis of second-order exchanges processes by NMR is difﬁcult

and the exchange process in 3 is sufﬁciently slow over the accessible

temperature range that factors other than exchange make a

signiﬁcant contribution to the linewidth, resulting in large errors

in the activation parameters that might otherwise be obtained.

We therefore restrict ourselves to a qualitative discussion of the

exchange process in 3.

Scheme 8 Proposed ﬂuxional pathway in 3[OTf].

Exchange in 3[OTf] is at least an order of magnitude slower

than in 3(Cl) and increasing the methanol content of the solvent

mixture signiﬁcantly depresses the rate. This solvent effect is

consistent with a dissociative process, however, suppression of the

rate by a coordinating solvent can also occur in an intermolecular

associative process since competition between the incoming ligand

and solvent for the ﬁfth metal coordination site can occur.⁶⁷

Moreover, solvation of liberated Cl^-might be expected to be

enhanced by an increase in methanol content of the solvent

mixture, which would be expected to result in an increase in rate

with increasing methanol content, contrary to the experimental

observation. Finally, if dissociation of Cl^-from Pd is rate

determining, then the inﬂuence of the counterion on the rate

is hard to explain. Therefore, we propose that the exchange is

associative and involves recoordination of the anion, Scheme 7.

This is consistent with the rate suppression by MeOH, and the

inﬂuence of the counterion; chloride is a “better” ligand for Pd(II)

than triﬂate or tetraﬂuoroborate,⁵¹so might be expected to re-

coordinate more readily.

Scheme 9 Proposed dynamic ﬂuxional pathways in 4 starting with

coordination of N¹.

Dynamic process in [Pd(j¹-Ph₂Ppy)₃Cl](X) (4(X)). Several dy-

1

namic processes can be detected in the ³¹P{ H} NMR spectra

of 4(Cl) and 4[OTf] (Supplementary Fig. 32–40, ESI†). At

temperatures below 233 K a very low energy process is seen that

desymmetrizes the molecule; the trans-PPh₂py ligands become

inequivalent. The extremely broad nature of the resonances at 193

K has prevented a detailed analysis of the dynamic process(es)

involved, however, the resonance of free Ph₂Ppy is sharp at both

213 and 193 K (Supplementary Fig. 33 and 34, ESI†) indicating

the low-temperature dynamic process is unlikely to involve free

Ph₂Ppy. At 193 K the resonances in the aromatic region of the

¹H NMR spectrum of 4 are also extremely broad. In the solid

state, p-stacking interactions are seen between the aromatic rings

of P¹and P³and between P²and P³and CH ◊ ◊ ◊ N interactions are

present between the pyridyl nitrogens of P¹and P²and a phenyl

C–H of P³, Fig. 6, inequivalencing P¹and P². Such p-stacking may

be responsible for the very low temperature dynamic process seen

Scheme 7 Proposed mechanism to equivalence the PN ligands in 3(Cl)

via recoordination of halide.

Although Scheme 7 can explain the equivalencing of the PN

ligands in 3(Cl), it does not explain the equivalencing in 3[OTf],

since the ligands remain distinct in the transition state, one

being trans to ligated chloride, the other trans to ligated triﬂate.

1

in the ³¹P{ H} NMR spectra of 4.

At higher temperatures, equivalencing of all three coordi-

nated PPh₂py ligands is seen (Supplementary Fig. 33–40, ESI†).

7930 | Dalton Trans., 2010, 39, 7921–7935

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ligands are trans to each other, a geometry we do not observe

in all other “monodentate–chelate” complexes reported here. The

observed regiospeciﬁcity in exchange with free PPh₂py can thus

be readily accounted for. However, the rapid equivalencing of P¹

with P^2/3is not accounted for by pathway a of Scheme 9 since P¹

remains trans to Cl.

Compare now Scheme 9 pathway b and Scheme 10. In Scheme 9,

coordination of the pyridyl nitrogen of P¹is followed by two Berry

pseudo-rotations leading ultimately to equivalencing of P¹N and

P³N. However, this scheme violates the principle of microscopic

reversibility; the forward reaction starts with coordination of

the nitrogen of the unique PPh₂py ligand, whereas the reverse

reaction begins with coordination of the nitrogen of one of the

pair of equivalent PPh₂py ligands. In Scheme 10, however, the

reaction passes through a symmetrical intermediate/transition

state as required by the principle of microscopic reversibility.

Thus, coordination of the pyridyl nitrogen of P¹N accounts for

regioselective exchange of P^2/3N with free PPh₂py (Scheme 9,

Fig. 6 Crystal structure of the monocationic complex of 4 highlighting the

CH ◊ ◊ ◊ N (blue) and p-stacking interactions (red) between the phosphine

ligands.

Simulations conﬁrm that the resonance of the pair of equivalent

PPh₂py ligands broadens faster than that of the unique PPh₂py

ligand, contrary to the expectation for a simple intramolecular

exchange of P¹N with P^2/3N; regioselective intermolecular ex-

change processes must also be occuring. However, although the

resonances of free PPh₂py and of traces of 3 are seen to broaden

at each temperature, the spectral lineshapes of the resonances

of the PPh₂py ligands in 4 are very similar, regardless of the

presence or absence of free PPh₂py or 3, or when there is a

deﬁciency of PPh₂py, or when the counterion present is changed,

compare Supplementary Fig. 33–40, ESI†. These observations

indicate that the rate determining step in the equivalencing of the

PPh₂py ligands in 4 is an intramolecular process. We can then

suggest two scenarios for the dynamic processes in 4: in the ﬁrst,

rate determining co-ordination of the pyridyl nitrogen of P¹N

displaces either P²N or P³N, Scheme 9, pathway a (P³N is shown).

The PPh₂py liberated is then scrambled with any free phosphine

present. This process involves coordination of the nitrogen of the

unique PPh₂py ligand to the apical site at Pd. Movement of one of

the trans PPh₂py ligands, which should be kinetically favoured over

that trans to Cl, gives a trigonal-bipyramidal intermediate C and

ultimately gives 3, in which the P donor atoms P¹and P²are cis.

By contrast, in Scheme 10, which begins with coordination of the

pyridyl nitrogen of either P²N or P³N (P³N is shown) the chelate

ring tethers P³hindering displacement of P²if the Hall pathway is

followed. Thus, movement of P¹N, giving species D, is favoured.

Loss of P¹N from D would afford a chelate–monodentate species

in which the P donors of the chelating (P³) and monodentate (P²)

pathway a) whilst coordination of the pyridyl nitrogen of P^2/3

N

accounts for equivalencing of P^2/3N and P¹N, Scheme 10. If the rate

determining step in both processes is coordination of the pyridyl

nitrogen (or loss of P³N from D in the intermolecular process),

then the relative insensitivity of the rate to free phosphine and the

counterion is also explained.

1

Simulation of the variable-temperature ³¹P{ H} NMR spectra

of 4 is complicated since more than one dynamic process is oc-

curing over the temperature range studied and by the observation

that the position of equilibrium between 3 and 4 is temperature

dependent, the amount of 3 present increasing with temperature.

This results in a signiﬁcant low frequency shift of the coalesced

resonance at higher temperature, see Supplementary Fig. 32,

ESI†. It is not possible accurately to separate the variables of

temperature effect on the chemical shift, concentration of 3 and

the three exchange rates. Above 273 K, the simulations become

less reliable due to the increasing amount of 3 present. Despite

the complexity of this system we have attempted to simulate the

variable-temperature spectra of solutions of 4 containing excess

PPh₂py, which should displace the equilibrium between 3 + PPh₂py

and 4 in favour of 4 (Supplementary Fig. 41 and 42, ESI†).

The values obtained from these simulations have also been used

to simulate the spectra of a sample containing stoichiometric

PPh₂py (Supplementary Fig. 43, ESI†). As expected for associative

processes involving rate determining intramolecular coordination

of a pyridyl nitrogen, a slight suppression of rate by free PPh₂py is

observed (Supplementary Table 1, ESI†) and can be attributed to

competition for the apical site at Pd. Although the values obtained

for DS^‡and DH^‡(Supplementary Table 1, ESI†) can only be

regarded as approximate, we note that DS^‡for both processes

‡

is negative, average DS_intra= -22 J mol^-1K^-1, DS_inter= -19 J

mol^-1K^-1similar to the values obtained for 10 and as expected for

an associative process. Enthalpies of activation are also similar to

those for 10, again consistent with rate determining association of

a pyridyl nitrogen.

Conclusions

Pd-PPh₂py chelate complexes have been suggested as interme-

diates in palladium-catalysed alkoxycarbonylation of propyne.

However, prior to the work presented here, the preferred

Scheme 10 Proposed intramolecular ﬂuxional pathway in 4 starting with

coordination of N²or N³.

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coordination mode of Ph₂Ppy to Pd(II) was, almost universally,

found to be monodentate coordination through phosphorus. In

contrast to these earlier reports, the present work has shown that

stable chelating palladium(II) complexes of Ph₂Ppy are readily

prepared by the use of solvent systems such as MeOH or CH₂Cl₂–

MeOH, in which MeOH presumably solvates the counterion.

Protonation of the pyridyl nitrogen of monodentate coordinated

PN in these complexes by the strong acid present in such alkyne

methoxycarbonylation catalyst systems as proposed by Drent,^7,17

Tooze,²⁵Scrivanti,^6,27and Elsevier,²²is observed. However, in

the solvent systems used (i.e. MeOH) we have shown that

shifts were referenced to TMS following IUPAC guidelines.

Spectra of samples dissolved in non-deuterated solvents were

referenced via the solvent resonances. Chemical shift errors are

1

as follows; ³¹P 0.2 ppm, H 0.05 ppm. Coupling constant

1

errors are as follows; ³¹P 1.0 Hz, H 0.1 Hz. In reporting

NMR data the letters s, d, t etc. have their normal meaning. Mass

spectrometric analyses were run in ES+ (positive electrospray)

ionisation mode on a Micromass LCT instrument using a ‘Time

of Flight’ mass analyser. X-Ray structures were determined on a

Bruker Smart APEX CCD diffractometer at 100 K using graphite

˚

monochromated Mo-Ka radiation (l = 0.71073 A). Crystals were

2

such protonation does not preclude the formation of k -PPh₂py

coated with nujol and then mounted on a glass ﬁbre. Structures

were solved by direct methods and structure reﬁnements by full-

matrix least-squares were based on all data using F². All non-

hydrogen atoms were reﬁned anisotropically unless otherwise

stated. Hydrogen positions were placed geometrically. Microanal-

yses were performed at the University of Liverpool using a Leeman

CE440 CHN Analyser or by Butterworth Laboratories Ltd.

All chemical reagents were purchased from Aldrich Chemical

Co., except [Pd₂dba₃],⁶⁸[Pd(Ph₂CN)₂Cl₂]⁶⁹and [Pd(Ph₂Ppy)₂dba]²⁴

which were prepared by published methods. ¹³CO (99.98%) was

purchased from Isotec Inc.

complexes.

2

The presence of severe strain in the Pd(k -PPh₂py) ring in

‘chelate–monodentate’ complexes might suggest Ph₂Ppy could

act as a hemilabile ligand furnishing a vacant coordination site

through de-coordination and re-coordination of the pyridyl ni-

1

trogen: variable-temperature ³¹P{ H} NMR spectroscopy reveals

2

that dynamic process do occur in Pd(k -PPh₂py). For example,

two stereospeciﬁc exchanges occur in 10, the lower energy process

equivalences the trans disposed ligands P²N and P³N, while a

higher energy process equivalences all three PN ligands (Schemes

5 and 6). Both mechanisms have a negative entropy of activation

consistent with an associative intramolecular exchange pathway.

Similarly, the dynamic processes in 4 can be explained by rate

determining coordination of a pyridyl nitrogen to palladium. The

dynamic exchange process observed in 3 is also probably associa-

tive but involves re-coordination of the counterion displacing the

[Pd(j²-Ph₂Ppy)Cl₂] (1). Ph₂Ppy (0.037 g, 0.14 mmol) was

added to a solution of [Pd(PhCN)₂Cl₂] (0.054 g, 0.14 mmol) in

dichloromethane (10 mL) and the mixture stirred for 30 min. The

resultant yellow solution was reduced to half volume in vacuo and

diethyl ether was added to precipitate an impure yellow solid. The

yellow solid, comprising a mixture of 1 with 2a/b from which pure

1 could not be isolated, was ﬁltered through a sinter, washed with

diethyl ether (3 ¥ 20 mL) and dried in vacuo to afford 0.054 g of

2

chelating nitrogen. Thus, although “Pd(k -PPh₂py)” complexes

may well be present in catalyst systems using Ph₂Ppy as a ligand,

it is not certain that the catalytic reaction will proceed via ligand

hemilability. Indeed, when weakly coordinating ligands, such as

methanesulfonate are present, it is these, and not the nitrogen of

the chelating Ph₂Ppy ligand, which are the more labile despite the

strain in the chelate ring.

1

the solid mixture. ³¹P{ H} NMR in CH₂Cl₂: d(P) -61.6 (s, 1); 29.0

(s, 2a); 22.8 (s, 2b).

[Pd(j¹-Ph₂Ppy)₂Cl₂] (2a/b). Ph₂Ppy (0.308 g, 1.17 mmol) was

added to a solution of [Pd(COD)Cl₂] (0.157 g, 0.56 mmol) in

dichloromethane (10 mL) and the mixture stirred for 30 min. The

resultant yellow solution was reduced to half volume in vacuo and

diethyl ether was added to precipitate the product as a yellow solid.

The yellow solid was ﬁltered through a sinter, washed with diethyl

ether (3 ¥ 20 mL) and dried in vacuo. Yield: 0.358 g, 91%. Anal.

Calc. for C₃₄H₂₈N₂P₂Cl₂Pd: C, 58.02; H, 4.01; N, 3.98. Found: C,

Experimental

General methods and procedures

All manipulations involving solutions or solids were performed

under an atmosphere of nitrogen using standard Schlenk

techniques. All solvents were dried and distilled under nitro-

gen following standard literature methods; i.e. CH₂Cl₂over

CaH₂, MeOH over Mg(OMe)₂, EtOH over Mg, THF over

Na/benzophenone, MeCN over CaH₂, n-hexane over Na and

Et₂O over Na/benzophenone. Deuterated solvents were degassed

by three freeze–pump–thaw cycles under vacuum in a liquid-

nitrogen bath, then nitrogen saturated and stored over activated

1

57.82; H, 4.02; N, 3.82%. MS (ES⁺, m/z): 667, [M - Cl⁺]. ³¹P{ H}

NMR in CH₂Cl₂: d(P) 29.0 (s, 2a); 22.8 (s, 2b).

trans-[Pd(j¹-Ph₂Ppy)₂Cl₂] (2b). A dichloromethane solution

of 2a/b was layered with diethyl ether. Crystals suitable for an

X-ray crystal structure determination were obtained upon slow

diffusion of the layers. Upon redissolution of the crystals in

1

CH₂Cl₂, the ³¹P{ H} NMR showed a mixture of cis and trans

isomers was present.

˚

4 A molecular sieves under nitrogen for at least 24 h prior to use.

Some compounds were obtained as impure oils and have been

characterized in solution only by NMR spectroscopy. No yield

or elemental analysis is reported for these compounds. Reactions

involving CO were carried out in a well ventilated fume-hood. All

other experiments have been carried out following standard safety

procedures.

[Pd(j²-Ph₂Ppy)(j¹-Ph₂Ppy)Cl]Cl (3(Cl)). A few drops of

MeOH were added to a suspension of 2a/b (0.025 g, 0.036 mmol)

in dichloromethane (2 mL) and the mixture shaken for a few

minutes until a clear yellow solution was obtained. Solvent was

removed in vacuo to give a yellow powder. Yield: 0.0238 g, 95%.

Anal. Calc. for C₃₄H₂₈N₂P₂Cl₂Pd: C, 58.02; H, 4.01; N, 3.98; Cl,

10.07. Found: C, 57.59; H, 3.98; N, 3.69; Cl, 10.33%. MS (ES⁺,

m/z): 667, [M⁺]. NMR in CH₂Cl₂–MeOH at 233 K: d(H) 8.77

(br, 1H, Ph₂Ppy, H⁶-py), 8.12 (br, 1H, J(HH) ~ 7 Hz, Ph₂Ppy,

1

³¹P{ H}, ³¹P, ¹³C{ H}, ¹³C and H NMR measurements were

performed on Bruker AMX2-200WB, AMX400, DPX400 or

Avance2-400 instruments using commercial probes. Chemical

7932 | Dalton Trans., 2010, 39, 7921–7935

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H⁴-py), 7.9 – 7.3 (24H, Ph₂Ppy); d(P) 44.4 (d, ²J(P¹–P²) = 33.0 Hz,

[Pd(j¹-Ph₂Ppy)₃Cl][OTf] (4[OTf]). Dichloromethane–MeOH

1

2

k -Ph₂Ppy), -44.0 (d, k -Ph₂Ppy).

(1 : 1) (2 mL) was added to a solid mixture of Ph₂Ppy (0.022 g,

2

1

0.084 mmol) and [Pd(k -Ph₂Ppy)(k -Ph₂Ppy)Cl]Cl, 3(Cl), (0.051 g,

0.068 mmol). Tl[OTf] (0.025 g, 0.071 mmol) was added to the

solution and the mixture stirred for 1 h. The resultant colourless

solution was ﬁltered from the precipitate of TlCl into 5 mL of

Et₂O. The resulting precipitate was washed three times with Et₂O

by decantation, collected by ﬁltration and dried in vacuo. The

precipitate analyzes as 4[OTf]·CD₂Cl₂Yield 0.068 g, 80%. Anal.

Calc. for C₅₃H₄₂D₂Cl₃F₃N₃O₃P₃PdS: C, 54.51; H/D, 3.97; N, 3.60.

Found: C, 54.69; H/D, 3.83; N, 3.80%. MS (ES⁺, m/z): 667, [M⁺

- PPh₂(C₅H₃N)]. NMR in CD₂Cl₂at 293 K: d(H) 8.39 (br, 3H,

Ph₂Ppy, H⁶-py), 7.22 (t, br, 3H, J(HH) ~ 5.4 Hz, Ph₂Ppy, H-py),

7.11 (dd, br, 3H, J(HH) ~ 7.25 Hz, Ph₂Ppy, H-py), 7.57–7.29 (30H,

Ph₂Ppy). NMR in CD₂Cl₂at 233 K: d(H) 8.33 (br, 1H, Ph₂Ppy,

H⁶-py), 8.29 (br, 2H, Ph₂Ppy, H⁶-py), 7.84–7.26 (29H, Ph₂Ppy),

7.22 (t, br, 2H, Ph₂Ppy, H-py), 7.17 (br, 1H, Ph₂Ppy, H-py), 7.11

(br, 1H, Ph₂Ppy, H-py), 7.00 (d, br, 2H, Ph₂Ppy, H-py), 6.25 (br,

[Pd(j²-Ph₂Ppy)(j¹-Ph₂Ppy)Cl][BF₄] (3[BF₄]). Ag[BF₄] (0.182 g,

0.93 mmol) was added to a suspension of 2a/b (0.657 g, 0.93 mmol)

in methanol–dichloromethane (1 : 1, 20 mL) and the resultant red

mixture stirred for 1 h. AgCl was removed by ﬁltration through

Celite and the resultant clear solution reduced to ca. 5 mL in vacuo.

MeOH (15 mL) was added to induce precipitation of 3[BF₄] as

yellow crystals which were suitable for an X-ray crystal structure

determination. Yield: 0.536 g, 76%.

Alternative procedure. Dichloromethane (2 mL) was added

to a solid mixture of Tl[BF₄] (0.029 g, 0.1 mmol) and 2a/b

(0.070 g, 0.1 mmol). The resulting solution was stirred for 1 h

then ﬁltered from the precipitate of TlCl into 25 mL of Et₂O.

The resulting precipitate was washed three times with Et₂O by

decantation, collected by ﬁltration and dried in vacuo. Anal. Calc.

for C₃₄H₂₈N₂P₂ClBF₄Pd: C, 54.07; H, 3.74; N, 3.71. Found: C,

53.88; H, 3.63; N, 3.66%. MS (ES⁺, m/z): 667, [M⁺]. NMR in

CH₂Cl₂–MeOH at 233 K: d(H) 8.77 (br, 1H, Ph₂Ppy, H⁶-py), 8.12

(t, 1H, J(HH) ~ 7 Hz, Ph₂Ppy, H⁴-py), 7.9–7.3 (24H, Ph₂Ppy); d(P)

1

2

1H, Ph₂Ppy, H-py); d(P) 33.2 (br, k -Ph₂Ppy), 25.6 (br, k -Ph₂Ppy)

[Pd(j²-Ph₂Ppy)(j¹-Ph₂Ppy)(MeCN)][BF₄]₂

(5). Excess

Ag[BF₄] (0.131 g, 0.67 mmol) was added to a solution of 3(Cl)

(0.225 g, 0.32 mmol) in a 1 : 1 dichloromethane–MeOH (20 mL),

and MeCN was added (1 mL). The mixture was stirred in the

dark for 2 h, then ﬁltered through celite to remove AgCl and

unreacted Ag[BF₄]. The resultant yellow solution was reduced

to ca. 2 mL in vacuo and diethyl ether added to precipitate the

complex as a yellow oil. NMR in CH₂Cl₂at 193 K: d(P) 41.3 (d,

1

2

44.8 (d, k -Ph₂Ppy), -43.8 (d, J(P¹–P²) = 38.2 Hz, k -Ph₂Ppy);

d(F) -152.5.

[Pd(j²-Ph₂Ppy)(j¹-Ph₂Ppy)(Cl][MeSO₃] (3[MeSO₃]). Dichlo-

romethane (2 mL) was added to a solid mixture of Tl[MeSO₃]

(0.035 g, 0.1 mmol) and 2a/b (0.070 g, 0.1 mmol). The resulting

solution was stirred for 1 h then ﬁltered from the precipitate of

TlCl into 25 mL of Et₂O. The resulting precipitate was washed

three times with Et₂O by decantation, collected by ﬁltration and

dried in vacuo. NMR in CH₂Cl₂–MeOH at 233 K: d(H) 8.77 (br,

1H, Ph₂Ppy, H⁶-py), 8.12 (t, br, 1H, J(HH) ~ 7 Hz, Ph₂Ppy, H⁴-

1

2

²J(PP) = 38.3 Hz, k -Ph₂Ppy), -46.3 (d, k -Ph₂Ppy).

[Pd(j¹-Ph₂Ppy)₂(p-benzoquinone)]

(7). Dichloromethane

(20 mL) was added to a solid mixture of [Pd(dba)₂] (1.00 g,

1.74 mmol), 2-pyridyldiphenylphosphine (0.98 g, 3.72 mmol) and

p-benzoquinone (0.188 g, 1.74 mmol) and the mixture stirred

under N₂for 3 h. The resultant red solution was ﬁltered and

the ﬁltrate concentrated to ca. 2 mL. Petroleum ether (15 mL)

was then added to precipitate the product as a red powder. The

powder was collected by ﬁltration, washed with petroleum ether

(3 ¥ 20 mL) and dried in vacuo. Yield: 0.54 g, 79%. Crystals

suitable for X-ray structure determination were obtained by slow

diffusion of petroleum ether into a dichloromethane solution of 7

(0.05 g, 0.067 mmol). Anal. Calc. for C₄₀H₃₂N₂O₂P₂Pd: C, 64.83;

H, 4.35; N, 3.78. Found: C, 64.67; H, 4.24; N, 3.52%. NMR in

CH₂Cl₂at 293 K: d(P) 30.2 (s).

2

py), 7.9–7.3 (24H, Ph₂Ppy); d(P) 44.7 (d, J(P¹–P²) = 38.2 Hz,

1

2

k -Ph₂Ppy), -43.8 (d, k -Ph₂Ppy). d(¹⁹F) -153.03 ppm. MS (ES⁺,

m/z): 667, [M⁺].

[Pd(j²-Ph₂Ppy)(j¹-Ph₂Ppy)(Cl][OTf]

(3[OTf]). Dichloro-

methane (2 mL) was added to a solid mixture of Tl[OTf] (0.035 g,

0.1 mmol) and 2a/b (0.070 g, 0.1 mmol). The resulting solution

was stirred for 1 h then ﬁltered from the precipitate of TlCl

into 25 mL of Et₂O. The resulting precipitate was washed three

times with Et₂O by decantation, collected by ﬁltration and dried

in vacuo. Anal. Calc. for C₃₅H₂₈ClF₃N₂O₃P₂PdS·0.5CD₂Cl₂: C,

49.52; H, 3.51; N, 3.25. Found: C, 49.32; H/D, 3.40; N, 3.25%.

NMR in CH₂Cl₂–MeOH at 233 K: d(H) 8.77 (br, 1H, Ph₂Ppy,

H⁶-py), 8.12 (t, 1H, J(HH) ~ 7 Hz, Ph₂Ppy, H⁴-py), 7.9–7.3 (24H,

[Pd(j²-Ph₂Ppy)(j¹-Ph₂Ppy)(MeSO₃)][MeSO₃]

(8(MeSO₃)).

MeSO₃H (14.4 mL, 0.22 mmol) was added to a solution of

7 (0.082 g, 0.11 mmol) in dichloromethane (2 mL) to give a

yellow–orange solution. Attempts to isolate 8 by solvent removal

in vacuo resulted in the formation of a yellow oil.

1

Ph₂Ppy); d(P) 44.6 (d, ²J(P¹–P²) = 38.1 Hz, k -Ph₂Ppy), -43.9 (d,

2

k -Ph₂Ppy). MS (ES⁺, m/z): 667, [M⁺].

[Pd(j¹-Ph₂Ppy)₃Cl](Cl) (4(Cl)). Ph₂Ppy (0.058 g, 0.22 mmol)

2

1

was added to a solution of [Pd(k -Ph₂Ppy)(k -Ph₂Ppy)Cl]Cl, 3(Cl),

(0.156 g, 0.22 mmol) in dichloromethane–MeOH (1 : 1) (20 mL)

and stirred for 1 h. The resultant yellow solution was reduced

to ca. 1 mL in vacuo. Crystals of 4(Cl) suitable for an X-ray

crystal structure determination were obtained by slow removal

of solvent from a saturated dichloromethane–MeOH solution at

reduced pressure. NMR in CD₂Cl₂at 293 K: d(H) 8.39 (br) (3H,

Ph₂Ppy, H⁶-py), 7.22 (br t, 3H, J(HH) ~ 5.4 Hz, Ph₂Ppy, H-py),

7.11 (dd, br, 3H, J(HH) ~ 7.25 Hz, Ph₂Ppy, H-py), 7.57–7.29 (30H,

[Pd(j²-Ph₂Ppy)(j¹-Ph₂Ppy)X][X] (8[X]). (X = MeSO₃, OTf,

CF₃CO₂). MeSO₃H (13 mL, 0.20 mmol) was added to a solution of

[Pd(OAc)₂] (0.022 g, 0.1 mmol) and Ph₂Ppy (0.053 g, 0.2 mmol) in

dichloromethane (2 mL) to give a yellow–orange solution, which

was stirred for 1 h, then ﬁltered by cannula into 25 mL of Et₂O.

The resulting precipitate was washed three times with Et₂O by

decantation, collected by ﬁltration and dried in vacuo. The triﬂate

and triﬂuoroacetate complexes were prepared analogously.

8[MeSO₃]: NMR in CH₂Cl₂at 193 K: d(H) 9.17 (br, 1H, Ph₂Ppy,

H⁶-py), 8.25 (t, br, 1H, J(HH) ~ 7 Hz, Ph₂Ppy, H⁴-py), 7.98–7.11

1

2

Ph₂Ppy). At 233 K: d(P) 33.1 (br, k -Ph₂Ppy), 25.7 (br, k -Ph₂Ppy)

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Dalton Trans., 2010, 39, 7921–7935 | 7933

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2

1

(24H, Ph₂Ppy); d(P) 41.2 (d, J(P¹–P²) = 29.8 Hz, k -Ph₂Ppy),

Resonances attributable to acetic acid and water integrating as

1 : 2 : 1 (9 : AcOH : H₂O) were observed in the ¹H NMR spectrum.

Anal. Calc. for C₃₈H₃₈B₂F₈N₂O₅P₂Pd: C, 48.31; H, 4.05; N, 2.97.

Found: C, 47.39; H, 3.95; N, 3.4%. NMR in CDCl₃at 293 K:

d(H) 9–6.7 (PPh₂py, 26H), 2.51 (CH₃CO₂H + H₂O, 4H), 2.18

2

-40.0 (d, k -Ph₂Ppy). 8[X] (X = OTf, CF₃CO₂) were prepared

similarly.

8[OTf]: Calc. for C₃₆H₂₈F₆N₂O₆P₂PdS₂: C, 46.44; H, 3.03; N,

3.01. Found: C, 46.84; H/D, 3.40; N, 3.17%. NMR in CH₂Cl₂at

193 K: d(H) 8.7 (br, 1H, Ph₂Ppy, H⁶-py), 8.35 (br, 1H, J(HH) ~

2

(CH₃CO₂H, 6H) ; in CH₂Cl₂at 193 K: d(P) -42.9 (br, k -Ph₂Ppy).

7 Hz, Ph₂Ppy, H⁴-py), 8.07 = 7.13 (24H, Ph₂Ppy); ³¹P{ H} d(P)

1

[Pd(j²-Ph₂Ppy)(j¹-Ph₂Ppy)₂][MeSO₃]₂(10). Methanol (2 mL)

was added to a solid mixture of [Pd(OAc)₂] (0.116 g, 0.52 mmol)

and Ph₂Ppy (0.409 g, 1.55 mmol). The resultant yellow solution

turned red on addition of MeSO₃H (67 mL, 1.04 mmol). A red oil

was obtained upon removal of the solvent in vacuo. Anal. NMR

in MeOH at 293 K: d(H) 8.26 (br, 3H, Ph₂Ppy, H⁶-py), 7.99 (t,

br, 3H, J(HH) ~ 5.4 Hz, Ph₂Ppy, H-py), 7.7–7.3 (33H, Ph₂Ppy).

NMR in MeOH at 183 K: d(H) 9–7 (33H, Ph₂Ppy); d(P) 44.7 (s,

1

2

40.8 (d, br, k -Ph₂Ppy), -39.3 (d, br, k -Ph₂Ppy).

8[CF₃CO₂]: NMR in CH₂Cl₂at 193 K: d(H) 8.5 (br, 1H, Ph₂Ppy,

H⁶-py), 8.25 (t, br, 1H, J(HH) ~ 7 Hz, Ph₂Ppy, H⁴-py), 7.9 = 7.2

2

1

(24H, Ph₂Ppy); d(P) 40.8 (d, J(P¹–P²) = 24.4 Hz, k -Ph₂Ppy),

2

-38.6 (d, k -Ph₂Ppy).

[Pd(j²-Ph₂Ppy)(j¹-Ph₂PpyH)X](X)₂(8¢(X)). One equivalent

of XH, (X = OTf, MeSO₃, BF₄) was added with stirring to a

solution 8[X] in a dichloromethane (2 ml) solution prepared as

described above. The resulting solution was stirred for 1 h, then

ﬁltered by cannula into 10 mL of Et₂O. The resulting precipitate

was washed three times with Et₂O by decantation, collected by

ﬁltration and dried in vacuo.

8¢[OTf]: NMR in CH₂Cl₂at 193 K: d(H) 9.31 (br, 1H, Ph₂Ppy,

H⁶-py), 8.93 (br, 1H, Ph₂Ppy, H⁴-py), 8.67 (t, br, 1H, J(HH) ~

7 Hz, Ph₂Ppy, H⁵-py), 8.35 (br) (1H, Ph₂Ppy, H⁴-py), 8.05–7.2

(22H, Ph₂Ppy); d(P) 26.4 (d, br,²J(P¹–P²) = 8 Hz, k -Ph₂Ppy),

1

2

1

br, k -Ph₂Ppy), 23.6 (d, br, J(trans-P²-P³) = 388 Hz, k -Ph₂Ppy)

2

-47.7 (d, br, k -Ph₂Ppy).

Variable-temperature NMR spectroscopic measurements for 3.

Typically, a 0.0178 mmol solution of 3(Cl) in dichloromethane

was used. For 3[OTf], one equivalent of Ag[OTf] was added to

the solution, and the supernatant liquid decanted from the AgCl

precipitate before transferring the solution to an NMR tube. The

1

variable-temperature ³¹P{ H} NMR spectra were recorded, and

1

simulated using gNMR5.1.

2

-45.0 (d, br, k -Ph₂Ppy).

Variable-temperature NMR spectroscopic measurements for

4. Typically, dichloromethane–MeOH (1 : 1) solutions of 4

(0.060 mmol) were prepared by addition of ~1 equivalent of

Ph₂Ppy to 3(Cl). To prepare the triﬂate salt 1 equivalent of Tl[OTf]

was added to the solution and the precipitate of TlCl removed by

8¢[MeSO₃]: NMR in CH₂Cl₂at 193 K: d(H) 9.48 (br, 1H, Ph₂Ppy,

H⁶-py), 9.11 (br, 1H, Ph₂Ppy, H⁴-py), 8.61 (br, 1H, J(HH) ~ 7 Hz,

Ph₂Ppy, H⁵-py), 8.35 (br, 2H, Ph₂Ppy, H⁴-py), 8.06–7.3 (22H,

1

2

Ph₂Ppy); d(P) 26.0 (br, k -Ph₂Ppy), -45.8 (br, k -Ph₂Ppy).

8¢[BF₄]: NMR in CH₂Cl₂at 193 K: d(H) 9.33 (br, 1H, Ph₂Ppy,

H⁶-py), 8.79 (br, 2H, Ph₂Ppy, H⁴-py), 8.5 (br, 1H, J(HH) ~ 7 Hz,

Ph₂Ppy, H⁵-py), 8.36 (br, 1H, Ph₂Ppy, H⁴-py), 8.09–7.2 (22H,

1

ﬁltration. The variable-temperature ³¹P{ H} NMR spectra were

recorded, and simulated using gNMR5.1.

1

2

Ph₂Ppy); d(P) 25.2 (k -Ph₂Ppy), -43.7 (k -Ph₂Ppy).

Determination of activation parameters for 10. Typically,

MeSO₃H (130 mg, 1.37 mmol) was added to a suspension of

(0.015 g, 0.064 mmol) palladium acetate in methanol (0.75 mL) to

give a red solution. This solution was transferred to an NMR tube

“Pd(Ph₂Ppy)₂(CF₃CO₂)₂” (2(CF₃CO₂)₂/8[CF₃CO₂]). CF₃-

CO₂H (15 mL, 0.22 mmol) was added to a solution of palladium

acetate (0.022 g, 0.1 mmol) and Ph₂Ppy (0.053 g, 0.2 mmol)

in dichloromethane (2 mL) to give a yellow–orange solution.

The solution was decanted into 3 mL Et₂O and the pale orange

precipitate collected by ﬁltration and dried. The precipitate

analyzes as [Pd(Ph₂Ppy)₂(CF₃CO₂)₂]·CH₂Cl₂Anal. Calc. for

C₃₈H₂₈Cl₂F₆N₂O₄P₂Pd: C, 49.08; H, 3.03; N, 3.01. Found: C,

48.67; H, 3.05; N, 4.02. NMR in CH₂Cl₂at 193 K: 2(CF₃CO₂)₂:

d(H) 8.58 (2H, H⁶-py), 8.13–7.17 (26H, PPh₂py); d(F) -74.5

1

and the variable-temperature ³¹P{ H} NMR spectra recorded,

simulated using gNMR5.1 and DS^‡, DH^‡and E_actwere obtained

from Eyring and Arrhenius plots respectively.

Spectral simulations. Simulation of variable-temperature

1

³¹P{ H} NMR spectra was performed using gNMR5.1. Coupling

constants and limiting chemical shifts were determined from a low

temperature, “static” spectrum. Coupling constants were treated

as constant in the simulations, however, pronounced temperature

effects on the chemical shift were noted. Chemical shifts were

therefore corrected iteratively in the simulations, using as a

criterion the assumption that chemical shift varies linearly with

temperature. Linewidths used in the simulations were estimated

from the linewidths of resonances not involved in a dynamic

process over the temperature range studied, and were treated as

constant. Association rates were taken to be twice the NMR rate

for intramolecular ﬂuxional processes as usual, and as equal to

the NMR rate in the intermolecular exchange process in 4(Cl).

1

(br, k -CF₃CO₂); d(P) 27.6 (br) (k -Ph₂Ppy); 8[CF₃CO₂]: d(F)

-74.0 (br, k -CF₃CO₂) -75.9 ([CF₃CO₂]^-); d(P) 40.8 (br, k -

Ph₂Ppy), -38.6 (br, k -Ph₂Ppy). NMR in CH₂Cl₂–MeOH at

1

2

1

193 K: 2(CF₃CO₂)₂: d(F) -74.5 (br) (k -CF₃CO₂); d(P) 27.7

1

(br, k -Ph₂Ppy); 8[CF₃CO₂]: d(H) 8.49 (1H, H⁶-py), 8.25 (1H,

H⁶-py), 8.13–7.17 (27H, PPh₂py); d(P) 40.8 (d, J(PP) -24.5 Hz,

2

1

2

1

k -Ph₂Ppy), -38.6 (d, k -Ph₂Ppy); d(F) -74.0 (br, k -CF₃CO₂),

-75.6 ([CF₃CO₂]^-).

[Pd(j²-Ph₂Ppy)₂][BF₄]₂(9). HBF₄·Et₂O (27 mL, 0.20 mmol)

was added to a solution of [Pd(OAc)₂] (0.022 g, 0.1 mmol)

and Ph₂Ppy (0.053 g, 0.2 mmol) in dichloromethane (2 mL)

to give an immediate yellow–orange precipitate. The precipitate

was collected by ﬁltration, washed three times with Et₂O and

dried in vacuo. The precipitate analysed as 9·2CH₃CO₂H·H₂O.

Acknowledgements

The ﬁnancial support of EPSRC, grants RG/R37685 and

EP/C0005643, is gratefully acknowledged. K. J. S. acknowledges

7934 | Dalton Trans., 2010, 39, 7921–7935

This journal is

The Royal Society of Chemistry 2010

©

View Article Online

the award of an EPSRC DTA studentship. F. Al. M. thanks

CARA for a Fellowship. The authors are grateful to Prof. Pierre

Braunstein, Prof. Bob Tooze of SASOL European Research

Laboratory and Dr Graham Eastham of Lucite International for

helpful discussions.

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Article Doi

The synthesis of, and characterization of the dynamic processes occurring in Pd(ii) chelate complexes of 2-pyridyldiphenylphosphine

DOI: 10.1039/b918162h

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Full text of DOI:10.1039/b918162h

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