Heterolytic activation of dihydrogen by platinum and palladium complexes

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Heterolytic activation of dihydrogen by platinum and

palladium complexes†‡

Cite this: Dalton Trans., 2013, 42, 6495

Karina Q. Almeida Leñero,^a,eYannick Guari,^b,fPaul C. J. Kamer,*^a,c

Piet W. N. M. van Leeuwen,*^a,gBruno Donnadieu,^bSylviane Sabo-Etienne,^b

Bruno Chaudret,^bMartin Lutz^dand Anthony L. Spek^d

Wide bite angle diphosphine ligands were used to prepare [(diphosphine)M(2-(diphenylphosphino)-

pyridine)]²⁺complexes (M = Pd, Pt). Except for the ligand with the largest bite angle, 2-(diphenylpho-

sphino)pyridine coordinates in a bidentate mode leading to bis-chelate complexes. In the case of Xant-

phos (9,9-dimethyl-4,5-bis(diphenylphosphino)-xanthene, β_n= 111°) two types of complexes are formed,

in which 2-(diphenylphosphino)pyridine coordinates in a mono- or bidentate fashion, respectively. The

crystal structures of three of the Pt complexes were determined. The X-ray crystal structure of [(Xantphos)-

Pt(2-(diphenylphosphino)pyridine)]²⁺shows that Xantphos coordinates in a tridentate P,O,P fashion.

Under dihydrogen pressure, the pyridyl moiety in the platinum complexes can de-coordinate to provide a

vacant coordination site at the metal center. Furthermore it can act as an internal base to assist the het-

erolytic cleavage of dihydrogen. The reaction yields a platinum hydride with a protonated pyridine

moiety in close proximity to one another. The structure as well as the reactivity of the complexes towards

dihydrogen is governed by the steric requirements of the diphosphines. The crystal structure of [(dppf)-

PtH(2-(diphenylphosphino)pyridinium)](OTf)₂has been determined. Palladium complexes containing

DPEphos or Xantphos decompose under dihydrogen pressure. In the case of dppf slow heterolytic split-

ting of dihydrogen occurs to form the hydride complex [(dppf)PdH(2-(diphenylphosphino)pyridinium)]-

(OTf)₂which contains a protonated 2-(diphenylphosphino)pyridine ligand. In solution, this compound

slowly undergoes P–C bond cleavage of the 2-(diphenylphosphino)pyridine ligand to form [(dppf)Pd-

(PHPh₂)(η¹-C₅H₄NH)](OTf)₂. When the 6-methyl-2-pyridyldiphenylphosphine ligand is used, the reaction

of the palladium complex with dihydrogen is very fast and the hydride complex immediately rearranges

to the diphenylphosphino compound resulting from P–C bond cleavage.

Received 9th October 2012,

Accepted 7th January 2013

DOI: 10.1039/c3dt32395a

www.rsc.org/dalton

Introduction

The activation of HX molecules (X = H, SiR₃) by transition

^aVan’t Hoﬀ Institute for Molecular Sciences, Universiteit van Amsterdam,

Nieuwe Achtergracht 166, 1018 WVAmsterdam, The Netherlands.

E-mail: pcjk@st-andrews.ac.uk, PvanLeeuwen@iciq.es

metal complexes is a key step in the mechanism of many cata-

lytic reactions.^1–3It can either occur via oxidative addition

involving a sigma-complex or not, for example in catalytic

hydrogenation, hydroformylation or hydrosilylation, or via

heterolytic cleavage as in processes such as hydrogen meta-

bolism,^4–6nitrogen fixation⁷or D₂/H⁺exchange (Fig. 1).^8–10

The heterolytic activation of HX is usually achieved using

metal centers in medium or high oxidation states and requires

the assistance of a Brønsted base. Some ligands act as intra-

molecular bases to assist the splitting of H₂, which has been

reported for complexes containing sulfides,¹¹thiolates,^8a,12

amines,¹³or nitrosyl groups.¹⁴During the cleavage of the HX

bond the main interaction involved is σ donation from HX to

an empty d orbital of the metal. Another important interaction

that sometimes occurs prior to the H–X bond splitting is a

proton–hydride interaction, or dihydrogen bonding.^15–18The

^bCNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne,

BP 44099, F-31077 Toulouse Cedex 4, France

^cEaStCHEM, Department of Chemistry, St. Andrews University, St. Andrews,

Scotland, UK

^dBijvoet Center for Biomolecular Research, Crystal and Structural Chemistry,

Utrecht Univerity, Padualaan 8, 3584 CH Utrecht, The Netherlands

^eShell Technology Centre Thornton, P.O. Box 1, Chester CH1 3SH, UK

^fUniversité Montpellier II, Place Eugène Bataillon, 34095 Montpellier, France

^gInstitute of Chemical Research of Catalunia (ICIQ), Av. Piasos Catalans 16,

43007 Tarragona, Spain

†Dedicated to Professor David Cole-Hamilton, a great scientist and even greater

friend.

‡Electronic supplementary information (ESI) available: Dihydrogen bonding and

hydrogenation studies, CIF files giving crystal data for Pt1b, Pt1c, trans1d and

Pt2a-Me. CCDC 904331, 904332, 904333, 904334. For ESI and crystallographic

data in CIF or other electronic format see DOI: 10.1039/c3dt32395a

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Fig. 1 Two pathways for the activation of dihydrogen.

interaction between a weakly acidic proton (NH or OH) and a Pt) as well as their reactivity towards dihydrogen. The resulting

transition metal hydride has a substantial strength (3–7 kcal complexes contain both a hydride and a protonated pyridyl

mol⁻¹), which lies in the range observed in conventional moiety in close proximity to one another, which makes them

hydrogen bonds. Therefore it can influence the structure and potential candidates for dihydrogen bonding.

reactivity of metal hydride complexes. It has been shown that

dihydrogen bonded adducts can be formed prior to proton

transfer from HA to a metal hydride to form a dihydrogen

complex. This reaction is the reverse of the heterolytic

splitting of H₂; thus complexes exhibiting H⋯H interac-

tions can be considered as intermediates in the activation

process.^19,20

Results and discussion

Synthesis of [(diphosphine)M(2-(diphenylphosphino)-

pyridine)]²⁺complexes (1a–d)

Chart 1 shows the diphosphine ligands used for this study,

together with their calculated, so-called natural bite angles

(β_n), as defined by Casey et al.^32,33

The ancillary ligands (usually phosphines) have an impor-

tant influence on dihydrogen bond formation. Morris et al.

compared the strength of intramolecular IrH⋯H bonds in

complexes with PCy₃and PPh₃. The stronger dihydrogen bond

was formed in complexes containing PCy₃, which increases the

basicity of the hydride.²¹Crabtree, Eisenstein et al.^13a,bfound

that while PPh₃favors the heterolytic splitting of dihydrogen

by iridium complexes (assisted by a pendant amino group),

more basic alkylphosphines form an η²–H₂complex. In our

research group, a series of wide bite angle diphosphine

ligands has been developed. Much eﬀort has been devoted to

the understanding of the influence of diphosphine ligands on

the reactivity and catalytic performance of transition metal

complexes.^22–24

The dicationic platinum and palladium complexes contain-

ing a chelating 2-(diphenylphosphino)pyridine (Ph₂PPy) ligand

were prepared by reaction of the corresponding (diphosphine)-

MCl₂(M = Pt, Pd) with silver triflate, followed by addition of

2-(diphenylphosphino)pyridine (Scheme 1). When silver tri-

flate is added to a CH₂Cl₂solution containing the dichloride

precursor, a deeply colored solution is formed. After a few

minutes precipitation of AgCl is observed. Upon addition of

2-(diphenylphosphino)pyridine, the color changes immedi-

ately to yellow for the complexes containing Xantphos-type

ligands and orange (Pt) or purple (Pd) for the dppf analogs.

Complexes 1a–d (Pt and Pd) were characterized in solution

by multinuclear NMR spectroscopy. ³¹P NMR spectroscopic

data are summarized in Table 1. In most cases, the spectra of

the platinum and palladium complexes are very similar, and

they will be described together. The few diﬀerences will be dis-

cussed afterwards. The ³¹P{¹H} NMR spectra display an AMX

spin system (Fig. 2) for the cis isomer and an A₂X system for

the trans isomer. For complexes 1a–c only the cis isomer was

observed, whereas for complexes 1d containing the Xantphos

ligand, a mixture of the cis and trans isomers was obtained.

The AMX spin system is consistent with a structure in which

the diphosphine is coordinated in a cis fashion and 2-(di-

phenylphosphino)pyridine acts as a chelating ligand. Biden-

tate coordination of 2-(diphenylphosphino)pyridine is

On the other hand, pyridylphosphine ligands exhibit a very

rich chemistry.^25,26Owing to the presence of both soft and

hard donor atoms, they can stabilize metal ions in several oxi-

dation states and geometries. When they act as chelating

ligands they can stabilize a catalyst precursor, while the nitro-

gen atom de-coordinates easily to provide a vacant site for sub-

strate binding and further reaction. Furthermore, the free

pyridyl moiety can act as an intramolecular base to assist the

heterolytic cleavage of H₂as shown by Jalón et al. for Ru–H₂

complexes.²⁷These characteristics are very important in cata-

lytic cycles, which often involve changes in the oxidation state

and the coordination number of the metal centers. The co-

operative nature of pendant base containing metal complexes

can lead to strongly improved performance in many catalytic

reactions.²⁸Noyori’s catalyst and the Shvo catalyst, both Ru,

are examples of such metal–ligand bifunctional catalysts.²⁹

A

simple ligand used as such is a secondary phosphine oxide

(SPO) ligand in Pt and Rh complexes.³⁰The importance of a

base is also illustrated by systems capable of metal free acti-

vation of dihydrogen such as carbenes and frustrated Lewis

pairs.³¹

We wish to report here the study of the influence of the

ancillary ligands on the structure of complexes of the type

[M(diphosphine)(2-(diphenylphosphino)pyridine)]²⁺(M = Pd,

Chart 1 Diphosphine ligands used and natural bite angles in degrees. ^aValue

from crystal structure. All values taken from ref. 22.

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Scheme 1 Synthesis of complexes (Pt, Pd)1a–d. Phenyl groups on phosphorus atoms have been omitted for clarity, P = PPh₂.

Table 1 ³¹P{¹H} spectroscopic data for complexes 1a–d

Complex

J_PA–PM(Hz) J_PA–PX(Hz) J_PM–PX(Hz) J_Pt–PA(Hz) J_Pt–PM(Hz) J_Pt–PX(Hz) δ_PA(ppm) δ_PM(ppm) δ_PX(ppm) δ_Pt(ppm)

Pt1a

Pt1b

Pt1c

Pt1d

Cis

Trans

Pd1a

Pd1b

Pd1d^a

Cis

Trans

Pt1a-Me^a

I

12.0

16.3

19.3

364

372

360

19.5

15.8

15.2

2737

2829

2772

3496

3536

3563

2058

2143

2142

24.0

15.2

12.8

9.16

−7.5

−6.6

−38.3

−39.2

−40.1

−4083

−4063

−4082

24.3

—

8.5

3.7

358

12.1

408

412

12.2

—

2800

2503

—

365

—

2118

4447

—

12.0

39.2

30.7

20.5

−6.5

—

41.7

20.1

−41.3

16.3

−42.3

−46.1

−4073

−3972

—

14

n.r.^b

—

4.9

—

388

23.1

n.r.^b

—

12.9

36.2

18.8

—

−45.1

35.1

—

12

354

15

389.4

21

19

12.2

2728

3461

—

3533

2538

—

2075

3764

—

20.9

16.4

25.1

5.0

23.2

41.1

−40.0

15.8

−40.8

n.d.^c

—

II

425

Pd1a-Me^a

n.r.^b

All spectra were measured in CD₂Cl₂at room temperature unless otherwise stated. ^aAt 233 K. ^bNon-resolved. ^cNot determined.

The chelating coordination of the 2-(diphenylphosphino)-

1

pyridine is usually also reflected in the H NMR spectrum of

the corresponding metal complexes, by a downfield shift of

the proton ortho to the nitrogen (H₆) in the pyridine moiety.

When the pyridine nitrogen coordinates, the resonance of H₆

shifts to higher frequencies compared to the free ligand^36,37

(above δ = 9 ppm for the chelate vs. δ = 8.72 ppm for the free

ligand in CDCl₃). Although for complexes 1a–d the resonance

for H₆is clearly separated from the rest of the aromatic

signals, it appears at a lower value than expected. A similar

trend was observed by James and co-workers,³⁸who studied

ruthenium complexes containing the 2-pyridylphosphines

PPh_3−x(Py)_x(x = 1–3). Tris-(2-pyridyl)phosphine acted as a tri-

dentate ligand with the phosphorus atom and two of the pyri-

dine nitrogens coordinated to ruthenium. They observed that

the signals for H₆in the coordinated pyridyl rings appeared

down-field compared to the resonance of H₆in the non-coordi-

nated pyridine. This may be due to the proximity of the phenyl

groups of the diphosphines to the pyridyl ring, causing an

unusual shielding of H₆. As will be discussed later, the phenyl

groups of wide bite angle diphosphines tend to embrace the

metal ion and cause important steric interactions with the

Fig. 2 Molecular structure of [(κ²-DPEphos)Pt(κ²-Ph₂PPy)][OTf]₂·3(CH₂Cl₂)

(Pt1b). The displacements ellipsoids are drawn at a 50% probability level. The

hydrogen atoms, the triﬂate anions and solvent of crystallization have been

omitted for clarity.

confirmed by the large up-field shift of P_Xwhich show reson- other coordinated ligands.

ances between δ = −38 and δ = −46 ppm. In the case of the

Although the purple color of Pd1a might suggest a Pd–Fe

platinum complexes, the value of the P–Pt coupling constant bond,³⁹no NMR or UV spectroscopic evidence for such an

(≈2000 Hz) is small compared to the values observed for the interaction was found. Indeed, both the ³¹P and the ¹H spectra

other two phosphorus nuclei (2500–3500 Hz). Both the chemi- are very similar for the platinum and palladium complexes.

cal shift and the J_PX–Ptcoupling are characteristic of a strained

The ³¹P spectrum of Pt1d displays a triplet (P_A) and a

doublet (P_X) with an intensity ratio of 2 : 1, in addition to the

four-membered ring.^34,35

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AMX system described above. These signals correspond to the the analogous platinum complex (Pt1d), neither exchange

trans isomer of Pt1d, in which 2-(diphenylphosphino)pyridine between the cis and trans isomers was observed at room temp-

acts as a monodentate ligand (Scheme 1). For compound trans- erature, nor did the ratio between the two isomers change with

Pt1d, the phosphorus atom of the 2-(diphenylphosphino)pyri- temperature.

dine ligand (P_X) displays a very large Pt–P coupling constant of

Analogs of complexes 1a using the bulkier ligand diphenyl-

J_PX–Pt= 4447 Hz. This value is commonly observed for phos- 2-(6-methyl-pyridyl)phosphine (Pt1a-Me and Pd1a-Me) were

phorus atoms trans to oxygen donor ligands.⁴⁰Furthermore, prepared following the same methodology as for 1a–d. Both

while the chemical shift of the ¹⁹⁵Pt nuclei in all cis complexes platinum and palladium complexes 1a-Me gave rise to very

does not vary by more than 20 ppm, δ_Ptof trans-1d is about broad signals at room temperature. Nevertheless, a diﬀerent

100 ppm higher than the average value of the former com- fluxional behavior is observed for Pt1aMe and Pd1aMe. In the

plexes (Table 1), indicating that there is an important diﬀer- case of platinum, two complexes can be observed upon lower-

ence in the environment of the platinum nucleus in cis and ing the temperature. At 233 K, two distinct sets of signals were

trans-Pt1d. This points to an interaction between the platinum observed in the ³¹P NMR spectrum in a 1 : 1 ratio. The first set

center and the oxygen of the Xantphos backbone, indicating corresponds to a complex analogous to Pt1a in which 2-(diphe-

that Xantphos coordinates as a tridentate ligand. This mer-P,O, nylphosphino)pyridine coordinates in a chelating fashion, cis-

P-κ³type of coordination for Xantphos has been observed in cat- κ²-P,N. Thus, its spectroscopic properties are very similar to

ionic Rh(^I) and Pd(II) complexes. It has been suggested that the those described for Pt1a (Table 1).

metal–oxygen interaction can stabilize the trans isomer with

The second set of signals is an ABX spin system for which

respect to the cis one in which this interaction is not both the chemical shifts and the P–Pt coupling constants indi-

present.^41–44For Xantphos and DPEphos both mer-κ³-P,O,P cate that 2-(diphenylphosphino)pyridine acts as a κ¹-P mono-

and fac-κ³-P,O,P coordinations have been observed recently dentate ligand. The ABX spin system rules out a trans structure

and this flexibility played an important role in the catalytic similar to trans-1d or a dimeric structure. The ¹H NMR spectra

reactivity of the complexes in hydroacylation.^45,46The mer-κ³-P, show two signals (δ = 1.59 and δ = 2.40 ppm) for the methyl

O,P coordination of Xantphos for Pt was confirmed by single groups of the pyridyl moiety, and a total of seven signals for

crystal X-ray diﬀraction, which will be discussed later.

the Cp protons in the dppf ligand. Two of the signals

The ¹H NMR spectrum of complex Pt1d displays two (4 protons in total) appear at δ = 3.26 and δ = 3.23 ppm and are

signals for the methyl groups in the backbone at δ = 1.77 and shifted up-field by more than 1 ppm, as observed for 1a. The

δ = 1.82 ppm. Integration of these signals indicates that the cis remaining signals (integrating for 12 protons) appear between

and trans isomers are present in a 1 : 1 ratio. For complex 1c, δ = 5.09 and δ = 4.27 ppm. This indicates that in both products

which exists as the cis isomer only, one signal (δ = 0.78 ppm) dppf is coordinated in a cis fashion (Scheme 2).

was observed for the CH₃groups in the backbone.

In the second product (Pt1aMe-II), 6-methyl-2-(diphenyl-

The corresponding palladium complex [(Xantphos)Pd(PPy)]- phosphino)pyridine is not chelating and the fourth coordi-

(OTf)₂(Pd1d) shows fluxional behavior. The ³¹P NMR spec- nation position is probably occupied by the triflate anion. The

trum displays only a broad singlet for the Xantphos ligand at IR spectrum shows two split bands for ν(SO₃) at 1260 and

room temperature, while the ¹H NMR spectrum shows a 1005 cm⁻¹. These bands have been previously assigned to

slightly broad singlet for all methyl groups and broad signals mono-coordinated sulfonates.⁴⁷Although triflates are known

for the aromatic protons. At low temperature, the singlet in the to be weakly coordinating anions, the IR data and the fact that

³¹P NMR spectrum becomes less intense and other signals the microanalysis of a sample containing both products is

start to emerge from the base line. At 233 K two sets of signals correct show that both compounds have the same composition

can be distinguished in the ³¹P NMR spectrum: a doublet and (see the Experimental section). The broad spectra observed at

a broad triplet with an intensity ratio of 2 : 1 (major product), room temperature indicate exchange between the two species.

and an AMX spin system (minor product) similar to that

In the case of palladium, complex Pd1a-Me shows fluxional

described above for complexes 1a–b. As discussed above, the behaviour at room temperature. The ³¹P NMR spectrum

two sets of resonances correspond to the cis and trans isomers reveals an exchange process between the two phosphorus

of Pd1d (Scheme 1). Thus the AMX spin system was assigned nuclei of dppf. The extra steric bulk introduced by the methyl

to the cis isomer containing chelating 2-(diphenylphosphino)- group induces a facile de-coordination of the pyridyl moiety,

pyridine. The second set of resonances corresponds to the

trans isomer, in which Xantphos coordinates in a tridentate

mer-κ³-P,O,P fashion.^35,43In the ¹H NMR spectrum at 233 K

three signals are observed for the methyl groups in the ligand

backbone, two of equal intensity for the cis isomer (δ = 1.9 and

δ = 1.6 ppm) and one (δ = 1.7 ppm) for the trans isomer. Inte-

gration of these signals shows a trans : cis ratio of 2.5, which

was confirmed by integration of the ³¹P spectrum. At 193 K,

the trans : cis ratio is 1.3, indicating that the formation of the

Scheme 2 Platinum complexes formed when 6-methyl-2-(diphenylphosphino)

pyridylphosphine chelate is favored at low temperature. For pyridine is used.

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probably assisted by the counterion or traces of free phosphine

in solution. Rotation in the five-coordinated species has a low

barrier and results in exchange of the positions of P_Aand P_M.

Upon lowering the temperature all signals broaden further,

but at 253 K they start to re-emerge from the base line. At

193 K the ³¹P NMR spectra show a well resolved AMX pattern

corresponding to [(dppf)Pd(κ²-6-methyl-2-(diphenylphosphino)-

pyridine)](OTf)₂, indicating that the dissociation or the iso-

merization of the square planar complex is slow on the NMR

timescale at this temperature.

Table 2 Selected bond distances (Å) and bond angles (°) for complexes

Pt1b–d

DPEphos (1b)

Sixantphos (1c)

Xantphos (1d)

Pt–P1

Pt–P2

Pt–P3

Pt–O1

Pt–N1

P1–C1

∠P1–Pt–P2

∠P2–Pt–P3

∠P3–Pt–N1

∠P1–Pt–N1

∠P2–Pt–N1

∠P1–Pt–P3

∠Pt–P1–C1

∠P1–C1–N1

∠P2–Pt–O1

∠P3–Pt–O1

∠P1–Pt–O1

2.324(3)

2.279(3)

2.337(3)

3.488(9)

2.112(8)

1.804(13)

99.32(10)

98.96(10)

92.3(3)

68.9(3)

168.1(3)

157.29(10)

83.3(4)

103.6(7)

—

2.3011(14)

2.2638(13)

2.3852(13)

2.725(3)

2.109(4)

1.818(5)

99.42(5)

94.95(5)

96.50(11)

69.08(11)

168.49(11)

164.89(5)

83.35(16)

103.4(3)

—

2.2387(7)

2.3171(5)

2.2976(5)

2.1889(18)

—

1.833(2)

96.90(2)

162.22(2)

—

A second species can be observed in the ³¹P NMR spectrum

of Pd1a-Me at room temperature. It displays a doublet at

−14.9 ppm and a triplet at 42.3 ppm, with a coupling constant

of 13.4 Hz. The chemical shifts and the coupling constant are

very similar to those reported by Sato et al. for [(η³-dppf)-

Pd(PPh₃)](BF₄)₂, which involves an Fe–Pd dative bond.³⁹There-

fore we propose that the second species in solution is [(κ³-P,Fe,

P-dppf)Pd(κ¹-6-methyl-2-(diphenylphosphino)pyridine)](OTf)₂.

This product is not involved in the exchange process men-

tioned above, so it displays sharp NMR signals at room temp-

erature. At 193 K, the signals for this product represent less

than 5% of the total intensity. Remarkably, Pd1a-Me displays

a very diﬀerent behavior from its platinum counterpart. For

the Pt complex, the species containing a chelating 6-methyl-

2-(diphenylphosphino)pyridine [(dppf )Pt(κ²-6-methyl-

2-(diphenylphosphino)pyridine)](OTf)₂and the one in which

the coordinated pyridyl has been replaced by the anion do not

exchange on the NMR time scale and separate sets of signals

for each isomer are observed. Furthermore, a complex with an

Fe–Pt bond was not detected at any temperature.

—

100.22(2)

110.78(11)

—

80.97(4)

82.37(4)

174.69(4)

—

For complexes Pt1b and Pt1c the coordination sphere of the

platinum atom consists of a cis coordinated diphosphine

ligand and chelating 2-(diphenylphosphino)pyridine. As com-

monly observed in complexes containing a chelating pyridyl-

phosphine, the bond angles involving the four-membered Pt1–

N1–C1–P1 ring show an important deviation from the ideal

90° value. The P1–Pt–N1 angles of 68.9(3)° for Pt1b and

69.08(11)° for Pt1c are slightly smaller than those reported for

other platinum complexes (70.8(5)° for [PtCl(κ²-Ph₂PPy)-

(PPh₂PPy)]⁺⁴⁹and 70.4(5)° in [PtMe(κ²-Ph₂PPy)(PPh₂PPy)]⁺).⁵⁰

The smaller bite angle of the 2-(diphenylphosphino)pyri-

dine ligand may be due to steric bulk of the diphosphine, com-

pared to the smaller methyl and chloride ligands. The

intrachelate angles of P1–C1–N1 (103.6(7)° in Pt1b and

103.4(3)° in Pt1c) and Pt–P1–C1 (83.3(4)° and 83.35(16)°,

respectively) show the same deviation from the ideal values as

observed in the above-mentioned complexes.

Solid state structures

The platinum complexes Pt1b, Pt1c and Pt1d were further

characterized by single crystal X-ray diﬀraction. Suitable crys-

tals were grown by slow diﬀusion of diethyl ether or hexanes

into dichloromethane solutions of the corresponding complex.

Selected bond distances and angles are presented in

Table 2. The structures found in the solid state are in agree-

ment with those proposed in solution from the NMR data dis-

cussed above. Thus, complexes with DPEphos (b) and

Sixantphos (c) form cis complexes, while for Xantphos (d) both

cis and trans isomers are present in solution. In the latter case,

only the trans isomer could be crystallized. The X-ray structure

of trans Pt1d confirms the P,O,P coordination mode of Xant-

phos, which was proposed based on the J_Pt–Pxcoupling con-

stant and the ¹⁹⁵Pt chemical shift. The solid state structures of

Pt1b and Pt1c show that there is considerable interaction

between the phenyl groups of the diphosphine ligand and

those of 2-(diphenylphosphino)pyridine. This interaction

becomes more important as the bite angle increases. In the

case of the Xantphos ligand 2-(diphenylphosphino)pyridine is

forced to act as a monodentate ligand. The “embracing eﬀect”

of the phenyl rings in wide bite angle diphosphines in palla-

dium allyl complexes was studied by van Haaren et al.⁴⁸

The Pt–N1 distance in Pt1b/c is almost 0.1 Å longer than

the one reported by Jain et al.⁵⁰and Farr et al.⁴⁹in the above-

mentioned platinum complexes, but in the same range as

those observed in [Ru(κ²-Ph₂PPy)(CO)₂Cl₂]⁵¹and [PdCl(κ²-

Me₂PPy)(PMe₂PPy)]⁺.⁵²

The platinum atom in Pt1b is shifted 0.124 Å away from the

least-squares plane determined by itself, the three phosphorus

atoms and the nitrogen atom, while the deviation is only

0.043 Å in Pt1c. Especially for the complex 1b (DPEphos), the

deviation from the coordination plane is much larger than the

0.017 Å or the 0.026 Å observed in [PtMe(κ²-Ph₂Ppy)(Ph₂PPy)]-

[BPh₄]⁵⁰and [Ru(η²-Ph₂PPy)(CO)₂Cl₂]⁵¹respectively. The larger

deviation from the coordination plane may be due to the pres-

ence of the additional chelating ligand, while in the two other

complexes reported the additional ligands are monodentate

and therefore can coordinate at an angle near the ideal 90°

value. Because DPEphos (b) and Sixantphos (c) have natural

bite angles larger than 100°, they will force a geometry with a

more open P2–Pt–P3 angle which leads to further distortion

around the Pt center. In fact, the P2–Pt–P3 angle is smaller

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(98.96(10)° in Pt1b and 94.95(5)° for Pt1c) than expected from

these wide bite angle diphosphines. Particularly for the Sixant-

phos complex (1c), the value of 94.95(5)° is at the lower end of

the flexibility range for this ligand (93–130°).⁵³In complex

Pt1b the P2–Pt–P3 angle is slightly smaller than the P–Pd–P

angle observed in palladium complexes containing DPEphos

prepared in our group (from 100.82° to 103.93°).^54,55Again,

the smaller bite angle may be caused by the steric interaction

between the phenyl rings of the diphosphine and those on

2-(diphenylphosphino)pyridine (Fig. 2 and 3).

The backbone of the DPEphos ligand is highly bent (Fig. 2),

the planes containing the aromatic rings form an angle of

102.9°. This conformation has been observed in other square

planar complexes of DPEphos.^54,55In the case of complex Pt1c

(Fig. 3), one of the aromatic rings of the Sixantphos backbone

is bent to the back, the angle between the planes formed by

(C4 to C9) and (C10 to C15) is 40.2°.

The Pt–O distance in Pt1b is 3.488(9) Å, but it is consider-

ably shorter in Pt1c (2.725(3) Å). The latter distance is only

slightly longer than the 2.714(3) Å reported for [Sixantphos-

Pd(4-C₆H₄CN)Br]⁵⁴in which the diphosphine ligand adopts a

trans geometry, and thus forces the oxygen atom close to the

metal. This is, to a lesser extent, also the case in Pt1c. Because

of the rigidity of Sixantphos, the Pt–O bond cannot be much

longer. A weak platinum–oxygen bond was proposed in a

Pt(^II)–β-diketonato complex in which the Pt–O distance was

2.796(6) Å.⁵⁶Similar Pd–O distances have been reported in

five-coordinated palladium(^II) complexes^57,58and thus the

short distance observed in Pt1c might also indicate the

presence of a weak Pt–O interaction.

Fig. 4 Molecular structure of [(mer-κ³-Xantphos)Pt(κ¹-P-Ph₂PPy)][OTf]₂·0.5

(H₂O) (trans1d). The displacement ellipsoids are drawn at a 50% probability

level. The hydrogen atoms, the triﬂate anions and solvent of crystallization have

been omitted for clarity.

influence of the aryl ligand compared to the phosphine in 1d.

The geometry around the metal is square planar. Because of

the rigidity of the Xantphos ligand, P2 and P3 are bent down

from the coordination plane and the P2–Pt–P3 angle is

162.22(2)°. The O–Pt–P1 angle (174.69(4)°) is closer to the

ideal value of 180°. The Xantphos backbone is also bent, the

angle between the aromatic rings (C4 to C9) and (O1, C9 to

C12) is 8.3°, while the angle between the latter and the (C11 to

C16) plane is 10.6°. The Pt–P2 and Pt–P3 distances are similar

to those observed in other trans coordinated Pt(^II) complexes.⁵⁹

The Pt–P1 bond length in Pt1d (2.2387(7) Å) is similar to those

observed in platinum complexes in which 2-(diphenylpho-

sphino)pyridine acts as a P-monodentate.^49,50This bond is

also considerably shorter than those in Pt1b/c.

In complex Pt1d (Fig. 4), Xantphos acts as a tridentate

ligand. The Pt–O distance is 2.1889(18) Å, which is slightly

shorter than the 2.1537(14) Å observed in [mer-κ³-Xantphos-

Pd(4-C₆H₄CN)]⁺.⁵⁴This is probably caused by the larger trans

Two factors play an important role in the coordination

chemistry of Pt1a–d; the natural bite angle of the diphosphine

ligands, and the steric constraints imposed by the chelation of

2-(diphenylphosphino)pyridine. While dppf and DPEphos can

easily coordinate in a cis fashion, the bite angle of Sixantphos

and particularly of Xantphos is too large for the ideal 90°

P–M–P angle in a cis coordinated square planar complex, but

neither of them is large enough for ideal trans coordination. In

fact, the crystal structure of Pt1d, as well as the other reported

structures with a trans coordinated Xantphos ligand, shows

that the P–M–P angle is always less than 165° with the phos-

phorus atoms bent away from the plane. Diphosphines with a

wide bite angle also have a large cone angle, which results in

less free space around the metal center for coordination of

other ligands. As the chelation of 2-(diphenylphosphino)pyri-

dine increases the steric bulk around the metal center, biden-

tate coordination of 2-(diphenylphosphino)pyridine becomes

less favoured as the bite angle increases. Furthermore, chela-

tion of 2-(diphenylphosphino)pyridine is unfavourable by the

intrinsic strain of the four-membered ring. Nevertheless,

Fig. 3 Molecular structure of [(cis-κ²-Sixantphos)Pt(κ²-Ph₂PPy)][OTf]₂·CH₂Cl₂

(Pt1c). The displacement ellipsoids are drawn at a 50% probability level. The

hydrogen atoms, the triﬂate anions and solvent of crystallization have been

omitted for clarity.

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2-(diphenylphosphino)pyridine acts as a chelating ligand in the H–P couplings are resolved and the hydride signal appears

Pt1a–d. Obviously, the Pt–N bond is strong enough to stabilize as a doublet of doublets of doublets, with the corresponding

the highly strained geometries in Pt1a–c, especially in the case Pt satellites. For complex Pt2d (Xantphos), the two cis H–P

of Pt1c in which the Sixantphos ligand is forced to adopt an coupling constants are not well resolved, the hydride signal

uncommonly small bite angle. When the steric hindrance in appears as a slightly broad doublet of triplets, with Pt satel-

Pt1d becomes more important, chelation of the 2-(diphenyl- lites. Protonation of the pyridine moiety was confirmed by the

phosphino)pyridine is more diﬃcult, and a mixture of cis and broad signal at about 15 ppm, with an integration ratio with

trans Pt1d is formed. Thus, the structure of complexes 1a–d is the hydride of 1 : 1. This signal broadens and shifts slightly to

a compromise between the steric hindrance induced by the higher fields on lowering the temperature.

bite angle of the diphosphine and the stabilization due to the

chelation of the 2-(diphenylphosphino)pyridine.

The new complexes show an ABX spin system in the ³¹P

NMR spectrum, in which the AB part is formed by the two

phosphines trans to one another and P_Xcorresponds to the

phosphorus trans to the hydride ligand. This assignment is

Heterolytic activation of H₂

Although the activation of dihydrogen by platinum and palla- confirmed by the relatively small P_X–Pt coupling constant

dium complexes and the formation of the respective metal (J_PX–Pt≈ 2200 Hz), which is normally observed for nuclei trans

hydrides have been proposed to play an important role in the to hydrides. At the same time, the increase of more than

mechanism of the palladium catalyzed hydroformylation^60,611000 Hz of the P_B–Pt coupling constant indicates a release of

and hydrogenation⁶²the number of hydride complexes which the ring strain. Complete spectroscopic data are shown in

1

have been prepared and isolated by this route⁶³is still limited. Tables 3 and 4. The H and ³¹P spectra did not change signifi-

Both Pt and Pd complexes with 2-(diphenylphosphino)pyridine cantly on lowering the temperature, apart from some broaden-

and ligands a–d were tested for their reactivity toward ing of the signals below 233 K. At 203 K, the N–H proton is no

dihydrogen.

longer observed.

Platinum complexes. Complexes Pt1b–d react with dihydro-

Surprisingly, complex Pt1a with the dppf ligand did not

gen under mild conditions (4 bar, 40 °C, 16 h) to aﬀord cat- react with dihydrogen, not even at 10 bar and 50 °C. The use

ionic platinum hydrides 2b–d according to Scheme 3. of a more basic solvent such as acetone led to complete

In this reaction, the coordinated pyridine moiety acts as a decomposition of the starting material without formation of

hemilabile ligand to provide a vacant coordination site as well the desired hydride product. When one equivalent of an exter-

as an internal base to assist the heterolytic splitting of the nal base (Et₂NH or DIPEA) was used, a hydride complex was

dihydrogen molecule.²⁷The new hydride complexes were formed together with the salt of the amine under the same

1

characterized by multinuclear NMR spectroscopy. The H and reaction conditions as for Pt1b–d. This complex could be pro-

³¹P NMR data are presented in Tables 3 and 4, respectively. tonated afterwards with HOTf to aﬀord 2a, in which the pyri-

The ¹H NMR spectrum of Pt2b–d shows a characteristic dine moiety is protonated. Although the bite angle and the

hydride signal between δ = −5 and δ = −8 ppm. The shape of basicity of dppf are similar to those of DPEphos, Pt1a and

the signal depends on the ligand used. For complexes Pt2b Pt1b show a strikingly diﬀerent reactivity towards the heteroly-

and Pt2c containing DPEphos and Sixantphos respectively, all tic activation of dihydrogen. More information on the

Scheme 3 Reaction of complexes Pt1b–d with dihydrogen. P = PPh₂.

Table 3 ¹H{³¹P} NMR data for the hydride complexes Pt2a–d

Complex

Ligand

Dppf

δ_H(ppm)

δ_N–H(ppm)

J_PA–H(Hz)

J_PB–H(Hz)

J_Px–H(Hz)

J_Pt–H(Hz)

Pt2a

−5.72

−5.61

−5.11

−6.81

−7.53

−8.14

15.1

14.35

11.7

10.5

13.5

13.0

13.7

14.4

18.5

22.5

—

16.5

14.7

n.r ^b

154

150

162

163

158

158.9

762

750

797

739

732

714

Pt2a-Me^acis

Pt2a-Me^atrans

Pt2b

DPEphos

Sixantphos

Xantphos

14.9

15.1

15.3

Pt2c

Pt2d

NMR spectra were measured at 300 MHz in CD₂Cl₂at room temperature, unless otherwise stated. ^aAt 233 K. ^bnot resolved.

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Table 4 ³¹P{¹H} NMR spectroscopic data for the hydride complexes Pt2a–d

Complex

J_PA–PB(Hz)

J_PA–PX(Hz)

J_PB–PX(Hz)

J_Pt–PA(Hz)

J_Pt–PB(Hz)

J_Pt–PX(Hz)

δ_PA(ppm)

δ_PB(ppm)

δ _PX(ppm)

Pt2a ^b

364

372

—

360

370

375

10.81

18.2

19.4

31.7

12.7

16.3

−8.8

−21.9

—

−10.5

−24.3

−19.6

3033

2750

2863

2905

3030

3045

2755

3079

—

2944

2922

2937

2310

2395

2255

2217

2286

2298

28.7

29.9

25.3

23.4

23.8

23.0

21.8

20.7

—

16.4

20.1

17.7

22.9

23.6

22.9

16.5

15.8

Pt2a-Me^acis

Pt2a-Me^atrans

Pt2b^b

Pt2c^b

Pt2d^b,c

All spectra were measured at 121.5 MHz in CD₂Cl₂at room temperature, unless otherwise stated. ^aAt 233 K. ^bChemical shifts and coupling

constants calculated with gNMR. ^cAt 160.5 MHz.

electronic properties of the metal center was sought by means

of ¹⁹⁵Pt NMR. The chemical shift of ¹⁹⁵Pt nuclei is sensitive to

the ligands present in the coordination sphere and is therefore

a useful probe of the electronic environment of the metal.^64,65

Nevertheless, as can be seen in Table 1, the ¹⁹⁵Pt chemical

shifts for complexes Pt1a and Pt1b are not significantly

diﬀerent, indicating that there are no large diﬀerences in the

Scheme 4 Products formed from the reaction of Pt2a-Me with dihydrogen.

coordination environment of these two complexes.

The explanation for the diﬀerent reactivity of Pt1a com-

pared to Pt1b may lie in the smaller cone angle of dppf

(229.7°) compared to that of DPEphos (240.2°).⁵⁴For allylic

alkylation reactions van Haaren et al. observed an important

diﬀerence in the activity and selectivity of the palladium cata-

lysts with dppf compared to the one with DPEphos.⁴⁸In the

case of Pt1b–d, the steric interaction between the phenyl

groups of the Xantphos-type ligands with 2-(diphenylphos-

phino)pyridine may labilize the Pt–N bond, whereas in Pt1a

the stronger coordination of the pyridyl moiety results in lack

of reactivity. The driving force for the formation of the hydride

complexes is probably the release of the ring strain in 2-(diphe-

nylphosphino)pyridine. Indeed, when the latter ligand was

replaced by 3-(diphenylphosphinomethyl)pyridine, which

forms a 6-membered metallacycle, the reaction with dihydro-

gen was very slow. The built-in base in complexes Pt1b–d does

not accelerate the reaction. The reaction of [(DPEphos)-

Pt(PPh₃)](OTf)₂with dihydrogen and one equivalent of an

external base (DIPEA) is as fast as the reaction of complex

Pt1b. This indicates that either the rate of de-coordination of

the pyridyl moiety is the rate-limiting step or that the equili-

brium concentration of the tricoordinate species is too low to

lead to an appreciable rate of H₂cleavage.

Complex Pt1a-Me (cis and trans) did react under the same

conditions as Pt1b–d to give a mixture of two hydride species.

The reaction mixture displays very broad signals in the ¹H and

the ³¹P NMR spectra at room temperature. On lowering the

temperature to 233 K relatively sharp multiplets appeared and

it was possible to identify three sets of peaks. One set corre-

sponds to unreacted starting material (Pt1a-Me-I) which is

present in about 10%. The second set of signals corresponds

to the major species in solution and it is characterized by a

doublet of doublets at δ = −5.61 ppm in the ¹H NMR spectrum

and an ABX spin system in the ³¹P NMR spectrum. The split-

ting pattern, coupling constants and chemical shifts are

similar to those observed for complexes Pt2a–d and therefore

this species was assigned to a hydride cis[dppfPtH{2-(6-CH₃-

C₅H₃NH)PPh₂}]²⁺(cis 2a-Me, Scheme 4). Protonation of the

pyridine moiety was confirmed by a broad peak at 14.3 ppm.

When the solution containing this mixture was filtered over

celite, the peaks for cis Pt2a-Me disappeared almost comple-

tely, which enabled the identification of the third product. The

¹H NMR spectrum of this product displays a sharp doublet of

triplets at δ = −5.11 ppm, while the ³¹P NMR spectrum shows

a doublet and a triplet coupled to one another. The relative

intensity of these signals is 2 : 1. Apart from the hydride peaks,

1

the H NMR spectrum shows also a signal at δ = 2.1 ppm for

the CH₃of the diphenyl-2-(6-methyl-pyridyl)phosphine ligand

and several signals between 4.5 and 3.5 ppm for the Cp

protons in dppf, confirming the presence of these two ligands

in the product. From these data the third product was assigned

to an isomer of Pt2a-Me in which dppf is coordinated in a

trans fashion while 6-methyl-2-(diphenylphosphino)pyridine is

trans to the hydride (Scheme 4).

Palladium. It is well known that palladium hydrides are

generally less stable than their platinum analogues.⁶⁶There-

fore, the reaction of the palladium complexes Pd1a–d towards

dihydrogen was investigated by NMR experiments at low temp-

erature and elevated pressure. As expected, the reaction of the

palladium complexes with dihydrogen was much faster than

those of the platinum complexes. When the bright orange sol-

ution containing [XantphosPd(κ²-2-(diphenylphosphino)pyri-

dine)](OTf)₂(Pd1d) was pressurized with 4 bar of H₂at room

temperature, the solution became immediately dark brown

and extensive precipitation of the palladium metal was

observed. Nevertheless almost no change was observed in the

¹H and ³¹P NMR spectra. The tube was subsequently heated to

40 °C and after one hour the peaks of the starting material

had completely disappeared. Several multiplets were observed

in the ³¹P NMR spectrum, but no hydride signal could be

detected. In order to avoid formation of the palladium metal

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Scheme 5 Reaction of complexes Pd1a and Pd1a-Me towards dihydrogen. The phenyl groups on the phosphorus have been omitted for clarity (P = PPh₂).

and the associated side reactions, the reaction was performed for the starting material together with a new triplet and a

at low temperature. The solution in the high pressure NMR doublet of doublets (δ = 22.63 and 32.22 ppm respectively).

tube was cooled to −60 °C before pressurizing with 4 bar of

dihydrogen, and indeed neither precipitation nor color change [(dppf)PdH(2-(diphenylphosphino)pyridinium)][OTf]₂

These signals were assigned to the new palladium hydride

Pd2a

was observed. The tube was then introduced into the pre- (Scheme 5). Complete conversion to Pd2a can be obtained by

cooled NMR probe at 213 K. No change in the spectra was carrying out the reaction in an autoclave under 10 bar of di-

observed, even when the solution was warmed to 273 K. After hydrogen for 65 hours.

increasing the pressure to 10 bar the reaction was followed at

In contrast to this very slow reaction, complex Pd1a-Me

253 K during 20 hours, but only after 15 hours various small reacts immediately under 3 bar of dihydrogen at room temp-

new peaks could be observed. The solution was stored at room erature. The initial dark brown solution turns bright red

temperature for 24 hours and the NMR spectra were measured within 15 minutes and small amounts of palladium metal are

again at 253 K. [cis-κ²-P,P-XantphosPd(κ²-2-(diphenylphos- formed. Reduction to metallic palladium is avoided if the reac-

phino)pyridine)][OTf]₂(Pd1d) remained the main species in tion is carried out at 0 °C. The ¹H and ³¹P NMR spectra

1

solution, but the H NMR spectrum showed very small peaks showed no starting material remaining. A small hydride signal

1

in the hydride region (δ = −9.1 ppm), together with several could be observed in the H NMR spectrum, but its intensity

broad peaks between δ = 13 and δ = 15 ppm. After a total time was very small compared with the CH₃and Cp protons, indi-

of 60 hours the starting material was still the major product, cating that the main product does not contain a hydride

but the hydride signal could no longer be observed and several ligand. The ³¹P{¹H} NMR spectrum displays an AMX splitting

new peaks appeared in the ³¹P NMR spectrum, which pattern, similar to that of Pd1a, but the disappearance of the

resembled that of the reaction carried out at 40 °C. Although it signal at δ = –43.6 ppm indicates that 2-(diphenylphosphino)-

was not possible to assign all peaks in the ³¹P NMR spectrum, pyridine is acting as a monodentate ligand. The proton-

the observation of multiplets at δ = 220 ppm points towards coupled ³¹P NMR spectrum reveals a strong P–H coupling in

the formation of phosphido bridged dimers. These dimers are the same range as the trans P–P coupling, giving rise to a

formed as a result of P–C bond cleavage in 2-(diphenylphos- pseudo-triplet. The ¹H NMR spectrum shows a doublet of

phino)pyridine (see below). Similar results were obtained from doublet of doublets centered at δ = 6.1 ppm, which becomes a

the reaction of [cis-κ²-P,P-DPEphosPd(κ²-2-(diphenylphos- singlet upon phosphorus decoupling (Fig. 5). The P–H coup-

phino)pyridine)](OTf)₂with dihydrogen, but in this case exten- ling constant (360 Hz) points to the presence of a direct P–H

sive decomposition can already be observed after 14 hours at bond. Furthermore, this product displays seven signals for the

1

room temperature. Apparently, complexes 1d and 1b are Cp rings of dppf and a broad singlet at δ = 12 ppm in the H

unable to react with dihydrogen below room temperature, but NMR spectrum. The ¹³C{¹H}{³¹P} NMR spectrum displays a

above this temperature the hydride product reacts further singlet at δ = 179.5 ppm which splits into a doublet of triplets

forming unidentified products.

when the spectrum is measured phosphorus-coupled (Fig. 5).

Diﬀerent reactivity was observed for complexes Pd1a and The values of the carbon–phosphorus coupling constants

Pd1a-Me bearing the dppf ligand. For Pd1a no reaction at all (J_CP= 125 (d) and 12.6 (pt) Hz) indicate that an aryl group is

was observed under 4 bar of dihydrogen over a period of coordinated to the palladium atom. After further characteriz-

1

8 hours. After 24 hours at 10 bar of H₂about 30% conversion ation by appropriate H–¹H, H–³¹P and H–¹³C HMQC corre-

to a hydride containing product was observed. The high field lation experiments, Pd3a-Me was identified as a palladium-aryl

region of the ¹H NMR spectrum shows a broad doublet of complex arising from the cleavage of the phosphorus–pyridyl

doublets at δ = −6.7 ppm, which resolves into a doublet of bond in coordinated 6-methyl-2-(diphenylphosphino)pyridine

doublets of doublets upon cooling to 233 K. Four new signals (Schemes 5 and 6). Protonation of the nitrogen was confirmed

for the Cp protons of dppf were observed between δ = 3.5 and by the broad singlet at δ = 12.2 ppm, which shows a long-range

δ = 5 ppm. A broad signal at 15.1 ppm was assigned to the pro- correlation with the protons of the methyl group. Related

tonated pyridyl moiety. The ³¹P NMR spectrum shows signals iridium pyridyl complexes were reported by Carmona et al.

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The hydride ligand in Pd2a (and the hypothetical Pd2a-Me)

can act as an intramolecular nucleophile towards phosphorus.

Because the nitrogen atom in Pd2a is protonated, the phos-

phorus–pyridinium bond is weakened, resulting in facile clea-

vage of this bond (Scheme 6). This mechanism would explain

the observation of a P–H bond in the final product and the

fact that the P–C(pyridyl) is selectively cleaved. Novak and co-

workers proposed a mechanism involving reductive elimin-

ation of a phosphonium salt to give a 14-electron Pd(0)

species, followed by oxidative addition of a P–C bond.⁷⁵

Although this mechanism will result in the same final product

as the nucleophilic substitution mechanism, the formation of

a dicationic phosphonium salt seems to be unlikely, due to the

positive charge in the pyridyl moiety.

This reaction pathway is in sharp contrast to that observed

for the platinum hydrides where the only decomposition

observed was the loss of dihydrogen to regenerate the starting

κ²-pyridylphosphine complex. One explanation for this diﬀer-

ence may be the stronger σ Pt–H bond compared to the Pd–H

bond, which makes the nucleophilic attack at the phosphorus

atom less likely in the platinum case.

Fig. 5 (A) Section of the ¹³C NMR spectrum showing the phosphorus coupling

on the 2 and 6 carbon atoms of the pyridyl moiety in Pd3a-Me. (B) ¹H NMR

spectra showing the P–H proton.

Solid state structure of Pt2a-Me

Crystals suitable for X-ray diﬀraction were obtained by slow

diﬀusion of diethyl ether into a dichloromethane solution of

Pt2a-Me. Fig. 6 shows the crystal structure of [(dppf)PtH{2-(6-

CH₃-C₅H₃NH)PPh₂}][OTf]₂. The most relevant bond distances

and angles are presented in Table 5. The four ligands around

the platinum are arranged in a plane, for which the maximum

deviation from the least-squares plane defined by Pt1–H1–P1–

P2–P3 is 0.089 Å. The geometry is strongly distorted from the

ideal 90° angles expected for a square plane. The more

Scheme 6 Proposed mechanism for the formation of complexes 3a and

3a-Me.

and the structures were better described as the pyridylidene

complexes.⁶⁷

While trying to crystallize the hydride complex Pd2a, the

light brown solution turned orange after several days at room

temperature. The NMR spectra of the orange-red solid

obtained after evaporation of the solvent showed very similar

features to those of Pd3a-Me. Although we did not observe any

sign of decomposition of Pd2a during its characterization in

solution, it slowly decomposes to form the aryl complex Pd3a.

No signs of the starting material (Pd1a) were detected in the

solution. This observation, together with the small amounts of

hydride species observed upon reaction of Pd1a-Me with di-

hydrogen, indicate that a palladium hydride is indeed an inter-

mediate in the conversion of Pd1a-Me to Pd3a-Me.

P–C bond cleavage in palladium phosphine complexes has

been widely studied because it is responsible for catalyst de-

activation as well as aryl redistribution in cross-coupling

reactions.^68–70Three main mechanisms have been proposed

for this reaction: oxidative addition of the phosphine-bound

aryl to form a terminal phosphido group (which will usually

react further to form dimeric compounds), nucleophilic attack

at a coordinated phosphorus, followed by addition of the

cleaved aryl group, and formation of metallophosphorane

complexes.⁷¹In this case the oxidative addition pathway can

be discarded because no phosphido bridged products were

observed by ³¹P NMR and there is no evidence of a Pd(IV)

species. Metallophosphoranes have been proposed as inter-

mediates especially when fluoride anions are involved,^72–74but

we are not aware of proven examples involving hydrides.

Fig. 6 Molecular structure of [(dppf )PtH{2-(6-CH₃-C₅H₃NH)PPh₂}][OTf]₂·CH₂Cl₂

(Pt2a-Me). The displacement ellipsoids are drawn at a 50% probability level.

The triﬂate anions, the hydrogen atoms (except for the hydride ligand) and

solvent of crystallization have been omitted for clarity.

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center, leaving less space for other ligands, and thus destabi-

lizing bidentate coordination of 2-(diphenylphosphino)pyri-

dine. If the diphosphine becomes too bulky as in the case of

Xantphos, P-monodentate coordination of 2-(diphenylphos-

phino)pyridine becomes favorable, resulting in a mixture of

cis- and trans-1d.

Table 5 Selected bond distances (Å) and bond angles (°) for complex Pt2a-Me

Bond distances (Å)

Bond angles (°)

Pt–H1

Pt–P1

Pt–P2

Pt–P3

P1–C1

1.64(6)

∠H1–Pt–P1

∠H1–Pt–P2

∠H1–Pt–P3

∠P1–Pt–P2

∠P2–Pt–P3

∠P1–Pt–P3

76.3(18)

174.2(19)

77.9(18)

105.98(4)

99.24(4)

153.57(4)

2.2737(11)

2.3410(12)

2.2897(11)

1.840(4)

A remarkable diﬀerence is observed in the reactivity of plati-

num and palladium complexes 1a–d towards dihydrogen and

in the stability of the product hydride complexes. Platinum

complexes react smoothly with dihydrogen under mild con-

ditions to produce stable hydride complexes Pt2a–d. In the

case of palladium, the reaction with dihydrogen under the

same conditions is fast, but the resulting hydride complexes

are unstable and react further to unidentified products. The

complexes bearing dppf show a remarkable diﬀerence in reac-

tivity. While complex Pt1a is unable to split dihydrogen

without the help of an external base, Pd1a is the only palla-

dium precursor that reacts with dihydrogen in a controlled way

to aﬀord a stable hydride complex. The more bulky ligand,

diphenyl-2-(6-methyl-pyridyl)phosphine, promotes the de-

coordination of the pyridyl moiety in both platinum and palla-

dium complexes (1a-Me), thus facilitating the reaction with

dihydrogen.

The platinum hydride complexes 2a–d are stable in the

solid state, but slowly lose dihydrogen in solution to regenerate

the starting material. For the palladium analogs a faster reac-

tion takes place: P–C bond cleavage of the pyridyl phosphine

ligand.

Complexes 2a–d contain a hydride as well as a proton

donor in the same molecule, but no dihydrogen bonds are

formed. It has been proposed that wide bite angle ligands

reduce the partial negative charge on the hydride ligand.³⁶In

the case of 2a–d, the decrease of the hydridic character

induced by the bite angle of the diphosphines results in a too

weak electrostatic interaction between the hydride and the pyr-

idinium proton.

important deviations are the H1–Pt–P1 angle (76.3(18)°) and

the H1–Pt–P3 angle (77.9(18)°). The bite angle of dppf (P2–Pt–

P3 is 99.24(4)°) is very similar to the 99.3(1)° observed in

[(dppf)PtCl₂],⁷⁴and falls within the standard range for Pt(II)–

dppf complexes.⁷⁶The preference of dppf for bite angles larger

than 90° as well as the steric interaction between dppf and the

6-methyl-2-(diphenylphosphino)pyridine ligand forces P1 and

P3 to bend towards the small hydride ligand, causing the

rather small P1–Pt–P3 angle (153.57(4)°). The Cp rings of dppf

are only slightly tilted, the angle between the two planes being

2.2°, which is relatively small for square planar complexes. The

Pt–P1 and Pt–P3 bond lengths are normal for this type of Pt

complex; the Pt–P3 distance of 2.2897(11) Å is in the range nor-

mally observed in (dppf)Pt(^II) complexes. In contrast, the Pt–

P2 bond is significantly longer (2.3410(12) Å) than other Pt–P

bonds observed for phosphines trans to a hydride ligand, for

example 2.304(3) Å in [PtH(SiPh₃)(PEt₃)₂]⁷⁷and 2.278(2) Å in

[PtH(CH₂CMe₃)(Cy₂PCH₂CH₂PCy₂)].⁷⁸The latter two com-

plexes contain alkyl phosphines, which might explain the

shorter Pt–P bond compared with the one in Pt2a-Me.

One of the phenyl rings of the 6-methyl-2-(diphenylphos-

phino)pyridine ligand (P1) is parallel to one of the phenyl

rings of P2. The angle between the rings (N1, C1–C5) and

(C18–C23) is only 5.5° and the distance between their cen-

troids is 3.493 Å, which probably indicates a π-stacking

interaction.

The Pt–H bond is relatively long (1.64(6) Å) compared with

the one reported for [PtH(CH₂CMe₃)(Cy₂PCH₂CH₂PCy₂)]⁶⁸

(1.56(5) Å), and to the Pt–H(terminal) distance determined by

neutron diﬀraction in [(dppe)PtH(μ-H)₂Pt(dppe)]⁺(1.610(2)).⁷⁹

Preliminary studies towards dihydrogen bonding and

hydrogenation catalysis have been performed but only low

activities were obtained (see ESI‡).

Experimental section

All manipulations were carried out under an argon atmosphere

using standard Schlenk or glove box techniques. All solvents

were dried and freshly distilled under nitrogen prior to use.

Dichloromethane was distilled from CaH₂, diethyl ether, tetra-

hydrofuran, hexanes and pentane were distilled from sodium/

benzophenone. DPEphos,²²Sixantphos,²²Xantphos,²²6-

Conclusions

methyl-2-(diphenylphosphino)pyridine,⁸⁰

(CH₃CN)₂PtCl₂,⁸¹

82

The coordination mode of hemilabile pyridylphosphine (COD)PdCl₂and Na[B(3,5-(CF₃)-C₆H₃)]⁸³were synthesized

ligands in a series of platinum and palladium [(diphosphine)- according to literature procedures. Dppf was purchased from

Pt(2-(diphenylphosphino)pyridine)]²⁺complexes is determined Aldrich Chemical Co. and used as received. 2-(Diphenylphos-

by the steric demands of the diphosphine ligand. Thus, stable phino)pyridine was purchased from Aldrich Chemical Co. and

bis-chelated complexes are formed with the smaller bite angle re-crystallized from hot hexanes prior to use. High pressure

diphosphines (dppf, DPEphos and Sixantphos) in spite of the reactions were carried out in home-made stainless steel auto-

strain imposed by the four-membered ring created by chelat- claves fitted with a glass liner, or in a Fisher-Porter bottle for

ing 2-(diphenylphosphino)pyridine. With increasing bite angle reactions up to 3 bar. NMR spectra were recorded on a Bruker

the phenyl rings of the diphosphine “embrace” the metal DPX 300, a Bruker DRX 300, a Bruker AMX 400 or a Varian

This journal is © The Royal Society of Chemistry 2013

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Innova 500. CD₂Cl₂was dried over CaH₂, vacuum transferred, Product 2, ABX spin system: 15.84 (dd, P_x, J_PxPA= 14.6 Hz, J_PxPB

degassed by three freeze–thaw cycles and stored over molecu- = 19.4 Hz, J_PXPt= 3764 Hz); 16.38 (AB, P_A, J_PAPB= 425 Hz, J_PAPt

lar sieves. Elemental analyses were performed by the Service 3461 Hz); 23.23 (AB, P_B, J_PBPt= 2538 Hz). IR (KBr pellet): ν(SO₃)

de Microanalyse du LCC-CNRS, Toulouse, France. X-ray diﬀrac- 1260 and 1005 cm⁻¹

tion studies were performed by the National NWO-CW Single Anal. Calcd for C₅₄H₄₄P₃O₆NS₂F₆PtFe: C 48.97; H 3.35.

=

.

Crystal Service Facility, Universiteit Utrecht, The Netherlands; Found: C 48.85; H 3.37.

or by the Service de Diﬀraction des Rayons X du LCC-CNRS,

Toulouse, France.

DPEphos(2-(diphenylphosphino)pyridine)platinum(^II)bis-

triflate (Pt1b)

dppf(2-(Diphenylphosphino)pyridine)platinum(^II)bis-triflate

(Pt1a)

This compound was prepared as described for 1a using

125 mg (0.157 mmol) of (DPEphos)PtCl₂and 100 mg

652 mg (0.759 mmol) of (dppf)PtCl₂and 511 mg (1.99 mmol) (0.392 mmol) of silver triflate in 10 mL of CH₂Cl₂. The reaction

of silver triflate were dissolved in 20 mL of dry CH₂Cl₂. The mixture changed from colorless to bright yellow. After 3 hours

color of the solution changes almost immediately from orange of stirring 45.4 mg (0.172 mmol) of 2-(diphenylphosphino)pyr-

to deep red and a white precipitate forms. The reaction idine in 5 mL of CH₂Cl₂were added via a cannula, and the

mixture was stirred for three hours with protection from light. reaction mixture turned pale yellow. The light gray solid

A solution of 251 mg (0.954 mmol) of 2-(diphenylphosphino)- obtained was recrystallized from CH₂Cl₂/diethyl ether to yield

pyridine in 10 mL of CH₂Cl₂was added via a cannula and the 1b as a white powder.

reaction mixture was stirred overnight. The formed AgCl was

Yield: 180 mg (90%). Crystals suitable for X-ray analysis

allowed to settle and the solution was filtered through celite. were obtained by layering a concentrated CH₂Cl₂solution of

The clear orange solution was evaporated in vacuum and the the pure product with 20 mL of hexane.

solid obtained was re-crystallized from CH₂Cl₂/diethyl ether to

yield Pt1a as a microcrystalline orange solid.

Yield: 950 mg (0.724 mmol), 95%.

¹H NMR (300 MHz, CD₂Cl₂, 295 K): 8.23 (m, 1 H, Py-H⁶),

7.98–6.80 (m, 39H, Ar), 6.66 (m, ¹H, Py-H⁵), 6.38 (m, ¹H, Py-

H³). ³¹P{¹H} NMR (121.5 MHz, CD₂Cl₂, 295 K): −39.2 (dd, P_X,

¹H NMR (300 MHz, CD₂Cl₂, 295 K): 8.22 (m, 1 H, Py-H⁶), J_PXPA= 372 Hz, J_PXPM= 15.8 Hz, J_PXPt= 2143 Hz); −7.5 (t, P_M,

8.03–7.11 (m, 33H, Ar), 5.2, 4.86, 4.77, 3.65 (s, 8H, Cp). ³¹P{¹H} J_PMPA/X= 16.3 Hz, J_PMPt= 3536 Hz); 15.8 (dd, P_A, J_PAPt= 2829

NMR (121.5 MHz, CD₂Cl₂, 295 K): −38.3 (dd, P_X, J_PxPA

364 Hz, J_PXPM= 19 Hz, J_PXPt= 2058 Hz), 9.16 (t, P_M, J_PMPA= 12 118.7 (C_I), 119.0 (d, CH, J_Cp= 7.5 Hz), 121.5 (q, CF₃, J_CF= 321.5

Hz, J_PMPt= 3496 Hz); 24.0 (dd, P_A, J_PAPt= 2737 Hz). ¹³C {¹H} Hz), 123.4, 125.9 (C_I), 125.9, 126.1 (CH), 127.5 (d, CH, J_Cp

NMR (125.7 MHz, CD₂Cl₂, 295 K): 67.1, 71.9 (d, C_I, Cp, J_Cp7.5 Hz), 126.5 (d, C_I, J_Cp= 38.1 Hz), 129.9, 130.8, 131.1, 133.2,

77 Hz), 78.1 (d, CH, Cp, J_Cp= 12 Hz), 77.2 (d, CH, Cp, 133.5, 133.7, 133.9, 134.4, 134.8, 135.6, 136.1, 137.7, 143.5 (d,

J_Cp= 9 Hz), 76.5 (d, CH, Cp, J_Cp= 11 Hz), 75.7 (d, CH, Cp, J_CpCH, Py, J_Cp= 5.0 Hz), 148.5 (d, CH, Py, J_Cp= 8.0 Hz), 158.9,

8 Hz), 117.7 (d, C_I, Ph, J_Cp= 59 Hz), 121.5 (q, CF₃, 159.1 (d, C–O, J_CP= 7 Hz), 170.4 (d, C_i, Py, J_Cp= 67.9 Hz).

J_CF= 321 Hz), 127.6 (d, C_I, Ph, J_Cp= 55 Hz), 129.2 (d, C_I, Ph, Anal. Calcd for C₅₅H₄₂P₃O₇NS₂F₆Pt: C 50.90; H 3.20. Found:

J_Cp= 70 Hz), 129.7, 129.9, 131.1, 131.3, 131.4, 133.2, 134.0, C 50.37; H 3.01.

=

Hz). ¹³C {¹H} NMR (75.5 MHz, CD₂Cl₂, 295 K): 118.0, 118.4,

=

134.1, 134.4, 134.9, 135.0 (CH, Ph), 143.5 (d, CH, Py, J_Cp

5 Hz), 148.3 (d, CH, Py, J_Cp= 13 Hz), 170.7 (d, C_i, Py, J_Cp

69 Hz).

=

Sixantphos(2-(diphenylphosphino)pyridine)platinum(^II)bis-

triflate (Pt1c)

Anal. Calcd for C₅₃H₄₂P₃O₆NS₂F₆PtFe: C 48.56; H 3.23. This compound was prepared as described for Pt1a using

Found: C 48.13; H 3.21.

502 mg (0.583 mmol) of (Sixantphos)PtCl₂and 374.8 mg

(1.46 mmol) of silver triflate in 25 mL of CH₂Cl₂, and adding

169 mg (0.642 mmol) of 2-(diphenylphosphino)pyridine in

10 mL of CH₂Cl₂. A yellow solid was obtained and recrystal-

dppf(2-(Diphenylphosphino)-6-methylpyridine)platinum(^II)bis-

triflate (Pt1a-Me)

This compound was prepared as described for Pt1a using lized from CH₂Cl₂/pentane to yield 1c as a white microcrystal-

250 mg (0.305 mmol) of (dppf)PtCl₂and 196 mg (0.762 mmol) line solid. Yield: 696 mg (0.515 mmol), 88%. Crystals suitable

of silver triflate dissolved in 20 mL of CH₂Cl₂and adding a for X-ray analysis were obtained by slow diﬀusion of pentane

solution of 93 mg (0.335 mmol) of 2-(diphenylphosphino)-6- into a concentrated CH₂Cl₂solution of the pure product, at

methylpyridine in

(0.275 mmol), 90%.

5

mL of CH₂Cl₂. Yield: 354 mg room temperature.

¹H NMR (300 MHz, CD₂Cl₂, 295 K): 8.20 (m, 1 H, Py-H⁶),

¹H NMR (300 MHz, CD₂Cl₂, 233 K): 8.10–6.67 (m, Ar, both 8.05–6.97 (m, 38 H, Ar), 6.82 (m, H, Py-H³), 0.77 (s, 6H, CH₃).

products); 5.03, 4.90, 4.75, 4.71 (s, 2H each, Cp prod. 2); 4.37, ³¹P{¹H} NMR (CD₂Cl₂at 295 K): −40.1 (dd, P_X, J_PXPA= 360 Hz,

4.29, 3.31 (s, 8H total, Cp product 1); 2.48 (s, 3H, CH₃prod. 2); J_PXPM= 15.2 Hz, J_PXPt= 2142 Hz); −6.6 (t, P_M, J_PMPA/X= 19.3 Hz,

1.72 (s, 3H, CH₃, prod. 1). ³¹P{¹H} NMR (121.5 MHz, CD₂Cl₂, J_PMPt= 3563 Hz); 12.8 (dd, P_A, J_PAPt= 2772 Hz). ¹³C {¹H} NMR

1

233 K): Product 1, AMX spin system: −40.0 (dd, P_X, J_PXPA

354 Hz, J_PXPM= 20.7 Hz, J_PXPt= 2075 Hz); 5.04 (dd, P_M, J_PMPA

12.2 Hz, J_PMPt= 3533 Hz); 20.9 (dd, P_A, J_PAPt= 2728 Hz). 321.5 Hz), 126.0 (d, C_I, C–O, J_Cp= 15.1 Hz), 126.2 (d, CH, J_Cp

=

(125.7 MHz, CD₂Cl₂, 295 K): −2.4 (s, Si–CH₃), 114.3 (d, C_I, J_Cp

66.5 Hz) 115.7 (d, C_I, J_Cp= 69.3 Hz), 121.5 (q, CF₃, J_CF

=

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7.8 Hz), 126.5 (d, C_I, J_Cp= 38.1 Hz), 127.3, 129.8, 129.9, 130.1, Ph); 79.5 (d, CH, Cp, J_CP= 12.7 Hz); 77.2 (d, CH, Cp, J_CP

131.0, 131.1, 133.0, 133.3, 133.9, 134.2, 134.6, 135.7, 139.5, 9.4 Hz); 76.7 (d, CH, Cp, J_CP= 9.3 Hz); 76.1 (d, CH, Cp, J_CP

=

139.8, 143.2 (d, CH, Py, J_Cp= 5.0 Hz), 148.9 (d, CH, Py, J_Cp

13.0 Hz), 170.7 (d, C_i, Py, J_Cp= 56.4 Hz).

=

10.6 Hz); 75.2 (d, CH, Cp, J_CP= 7.6 Hz); 75.1 (d, C_I, Cp, J_CP

66.3 Hz); 66.0 (d, C_I, Cp, J_CP= 58.5 Hz).

Anal. Calcd for C₅₇H₄₆P₃O₇NS₂F₆SiPt·CH₂Cl₂: C 49.0; H

3.21. Found: C 49.71; H 3.40.

Anal. Calcd for C₅₃H₄₂P₃O₆NS₂F₆PdFe: C 52.09; H 3.46.

Found: C 52.12; H 3.50.

During the crystallization of this compound, orange plates

were obtained by slow diﬀusion of pentane into a solution of

Pd1a in THF. X-ray analysis of the crystals revealed that they

Xantphos(2-(diphenylphosphino)pyridine)platinum(^II)bis-

triflate (Pt1d)

This compound was prepared as described for Pt1a using corresponded to [Pd(μ-2-(diphenylphosphino)pyridine)OTf]₂.

250 mg (0.296 mmol) of (Xantphos)PtCl₂and 190.2 mg

(0.740 mmol) of silver triflate in 15 mL of CH₂Cl₂, and adding

86 mg (0.330 mmol) of 2-(diphenylphosphino)pyridine in

dppf(2-(Diphenylphosphino)-6-methylpyridine)palladium(^II)-

bis-triflate (Pd1a-Me)

5 mL of CH₂Cl₂. A yellow solid is obtained and recrystallized This compound was prepared using the same procedure as

from CH₂Cl₂/ether to yield 1d as a light yellow powder. Yield: described for 1a using 200 mg (0.273 mmol) of dppfPdCl₂and

335 mg (0.251 mmol) 85%. Crystals suitable for X-ray analysis 176 mg (0.683 mmol) of AgOTf in 10 mL of CH₂Cl₂, and

were obtained by layering a concentrated CH₂Cl₂solution of 83.3 mg (0.301 mmol) of 2-(diphenylphosphino)-6-methylpyri-

the pure product with 10 mL of hexane.

dine in 5 mL of CH₂Cl₂. After working-up as for Pd1a, a dark

¹H NMR (300 MHz, CD₂Cl₂, 295 K): 8.44–6.12 (m, Ar, 40H); brown microcrystalline solid was obtained. Repeated crystalli-

1.95 (s, CH₃, cis prod.); 1.72 (s, CH₃, 1d-trans); 1.62 (s, CH₃, 1d- zation from CH₂Cl₂/pentane was necessary to obtain a pure

cis), 6H total for the three signals. ³¹P{¹H} NMR (121.5 MHz, product.

CD₂Cl₂, 295 K): 1d-cis: −41.3 (dd, P_X, J_PXPA= 358 Hz, J_PXPM

12.2 Hz, J_PXPt= 2118 Hz); −6.5 (dd, P_M, J_PMPA= 24.3 Hz, J_PMPt

=

Yield = 260 mg (0.210 mmol), 77%.

¹H NMR (300 MHz, CD₂Cl₂, 233 K): 7.93–7.00 (br. m, 33H,

3654 Hz); 12.0 (dd, P_A, J_PAPt= 2800 Hz). 1d-trans: 16.3 (t, P_X, Ar), 4.77 (s, 2H, Cp), 4.69 (s, 2H, H Cp), 4.33 (br. s, 4H, Cp),

J_PAPx= 12.1 Hz, J_PXPt= 4447 Hz), 39.2 (d, P_A, 2P, J_PAPt

=

1.61 (br. s, 3H, CH₃). ³¹P{¹H} NMR (121.5 MHz, CD₂Cl₂,

2503 Hz).

233 K): −43.6 (d, P_X, J_PXPA= 400 Hz), 25.0 (d, P_A), 40.3 (P_M). ¹³

C

=

Anal. Calcd for C₅₈H₄₆P₃O₇NS₂F₆Pt: C 52.18; H 3.47; N 1.05. {¹H} NMR (125.7 MHz, CD₂Cl₂, 193 K): 166.0 (d, C_i, Py, J_CP

Found: C 52.05; H 3.35.

66.4 Hz); 162.1 (d, C_i, Py, J_CP= 13.8 Hz); 141.9 (CH, Py); 134.4,

133.8, 133.5, 132.7, 131.0, 130.9, 131.3, 130.2, 129.4, 129.2,

dppf(2-(Diphenylphosphino)pyridine)palladium(^II)bis-triflate

(Pd1a)

128.7 (CH, Ar); 128.9, 119.1, 118.8 (C_I, Ar); 120.7 (q, CF₃, J_CF

=

320.7 Hz); 119.7, 119.3 (C_I, Ph); 79.1 (m, CH, Cp); 71.6 (d, C_I,

150 mg (0.205 mmol) of (dppf)PdCl₂and 132 mg Cp, J_CP= 74.4 Hz); 69.3 (d, C_I, Cp, J_CP= 59.8 Hz); 24.9 (CH₃).

(0.512 mmol) of silver triflate were dissolved in 10 mL of

This product contained small amounts of [(κ³-dppf)Pd(κ¹-

CH₂Cl₂in a Schlenk tube protected from light. The color of (2-(diphenylphosphino)-6-methylpyridine))](OTf)₂.

the solution changed immediately from orange to deep green

³¹P{¹H} NMR (121.5 MHz, CD₂Cl₂, 233 K): 42.3 (t, PPh₂Py,

and a white precipitate was formed. The reaction mixture was J_pp= 13.2 Hz); −14.9 (d, κ³-dppf).

stirred for 1 hour. The AgCl formed is allowed to settle and the

Anal. Calcd for C₅₄H₄₄P₃O₆NS₂F₆PdFe: C 52.47; H 3.59.

solution was filtered into a Schlenk flask containing a solution Found: C 52.67; H 3.62.

of 65 mg (0.246 mmol) of diphenylphosphino-2-pyridine in

DPEphos(2-(diphenylphosphino)pyridine)palladium(^II)bis-

triflate (Pd1b)

5 mL of CH₂Cl₂. The color of the solution changed from green

to purple. The reaction mixture was stirred for two hours and

the solution was filtered over celite. The deep purple solution 126 mg (0.176 mmol) of (DPEphos)PdCl₂were suspended in

was evaporated in a vacuum and the solid obtained was 15 mL of CH₂Cl₂and 113 mg (0.440 mmol) of silver triflate in

washed with diethyl ether (2 × 5 mL) to yield Pd1a as a purple 5 mL of CH₂Cl₂were added. The dark yellow suspension was

solid. The crude product was re-crystallised from CH₂Cl₂/Et₂O stirred overnight after which a bright orange solution had

to obtain pure Pd1a.

formed. The AgCl formed was allowed to precipitate and the

solution was filtered into a Schlenk flask containing 55.6 mg

Yield = 200 mg (0.164 mmol), 80%.

¹H NMR (300 MHz, CD₂Cl₂, 295 K): 8.08 (m, 1 H, Py-H⁶), (0.211 mmol) of 2-(diphenylphosphino)pyridine in 5 mL of

7.97–7.07 (m, 33H, Ar), 5.28 (s, 2H, αH Cp), 4.84 (s, 2H, βH CH₂Cl₂. The reaction mixture was stirred for 16 h. The same

Cp), 4.47 (s, 2H, βH Cp), 3.65 (s, 2H, αH Cp). ³¹P{¹H} NMR work-up procedure as for Pd1a was used to obtain Pd1b as a

(121.5 MHz, CD₂Cl₂, 295 K): −42.3 (dd, P_X, J_PXPA= 408 Hz, fluﬀy bright yellow solid. Yield = 181 mg (0.150 mmol), 85%.

J_PXPM= 14 Hz), 30.7 (dd, P_A, J_PAPM= 8.5 Hz), 41.7 (t, P_M). ¹³C

¹H NMR (300 MHz, CD₂Cl₂, 295 K): 9.2 (m, 1H, Py-H⁶),

{¹H} NMR (125.7 MHz, CD₂Cl₂, 295 K): 148.4, 142.3 (CH, Py); 8.05–6.72 (m, 40H, Ar), 6.56 (m, ¹H, Py-H³). ³¹P{¹H} NMR

134.7, 134.6, 134.5, 134.1, 133.8, 133.6, 133.5, 133.3, 130.9, (121.5 MHz, CD₂Cl₂, 295 K): −46.0 (dd, P_X, J_PXPA= 412 Hz),

130.8, 130.7, 130.6, 129.6, 129.5 (CH, Ar); 130.3, 129.8, 127.8, 20.05 (t, P_M), 20.49 (dd, P_A, J_PAPM= 3.7 Hz). ¹³C {¹H} NMR

127.5 (C_I, Ph); 121.2 (q, CF₃, J_CF= 321.1 Hz); 119.7, 119.3 (C_I, (125.7 MHz, CD₂Cl₂, 295 K): 169.1 (d, C_i, Py, J_CP= 86.1 Hz);

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159.2, 158.3 (d, C–O); 148.8 (d, CH, Py, J_CP= 12.2 Hz); 142.4 Cp); 4.61 (s, 2H, Cp); 4.30 (s, 2H, Cp); 3.62 (s, 2H, Cp); −5.72

(CH, Py); 139.9, 136.6, 135.7, 135.1 (CH, Ar); 134.8 (d, CH, J_CP

11.8 Hz); 134.5 (d, CH, J_CP= 10.9 Hz); 134.2, 134.0 (d, CH, J_CP

=

(s, 1H, hydride, J_HPA= 11.7, J_HPB= 18.45 Hz, J_HPX= 154 Hz

J_HPt= 762 Hz). ³¹P{¹H} NMR (121.5 MHz, CD₂Cl₂, 295 K): 22.9

2.9 Hz); 133.7 (d, CH, J_CP= 11.4 Hz); 133.3, 132.9, 132.3, 131.5 (P_X, J_PXPA= 10.81 Hz, J_PXPB= −8.8 Hz, J_PxPt= 2310 Hz); 21.8 (P_B,

(d, CH, J_CP= 4.6 Hz); 130.8 (d, CH, J_CP= 11.81 Hz); 130.3 (d, J_PAPB= 364 Hz, J_PBPt= 2755 Hz); 28.7 (P_A, J_PAPt= 3033 Hz). IR

CH, J_CP= 11.4 Hz); 130.1, 129.9, 129.6 (d, CH, J_CP= 12.2 Hz); ν(Pt–H) = 2078 cm⁻¹

128.9, 127.1 (d, CH, J_CP= 8.9 Hz); 126.0 (d, CH, J_CP= 8.4 Hz);

.

125.0 (d, CH, J_CP= 5.5 Hz); 127.4, 126.9, 125.9 (C_I); 125.6 (d, C_I,

dppf(2-(Diphenylphosphino)-6-methylpyridine)-

hydridoplatinum(^II)bis-triflate (Pt2a-Me)

J_Cp= 12.0 Hz); 121.5 (q, CF₃, J_CF= 321.5 Hz); 120.7 (d, C_I, J_Cp

=

12.5 Hz); 120.3, 120.1, 119.7, 119.2, 118.77 (C_I).

100 mg (0.075 mmol) of Pt1a-Me were dissolved in 10 mL of

CH₂Cl₂and introduced into a stainless steel autoclave under

Ar and purged 3 times with H₂. The autoclave was pressurized

to 4 bar of H₂and heated to 40 °C overnight. After cooling to

room temperature and reducing the pressure to 1 bar, the reac-

tion mixture was transferred to a Schlenk flask under 1 bar of

H₂. The orange solution was evaporated under vacuum and

the remaining solid was washed with 10 mL of pentane and re-

crystallized from CH₂Cl₂/Et₂O to aﬀord Pt2a-Me as a dark

orange solid. Yield: 93 mg (0.070 mmol), 93%.

Anal. Calcd for C₅₅H₄₂P₃O₇NS₂F₆Pd: C 54.76; H 3.51.

Found: C 55.01; H 3.62.

Xantphos(2-(diphenylphosphino)pyridine)palladium(^II)bis-

triflate (Pd1d)

This compound was prepared using the same procedure as

described for Pd1b using 231 mg (0.306 mmol) of Xant-

phosPdCl₂and 196 mg (0.764 mmol) of AgOTf and 96.6 mg

(0.367 mmol) of 2-(diphenylphosphino)pyridine in 5 mL of

CH₂Cl₂. The same work-up procedure as for 1a was used to

obtain an orange-yellow solid.

¹H NMR (300 MHz, CD₂Cl₂, 295 K): hydride region, 2a-Me

cis −5.61 (ddd, J_HPA= 10.5 Hz, J_HPB= 22.5 Hz, J_HPX= 150.0 Hz,

Yield = 362 mg (0.362 mmol), 90%.

J_HPt= 750 Hz). 2a-Me trans −5.11 (dt, J_HPA= 13.5 Hz, J_HPX

=

¹H NMR (CD₂Cl₂at 233 K, a mixture of cis and trans com-

plexes: 8.42–6.09 (m, 66H, Ar), 1.91 (s, 1.2H, CH₃cis), 1.72 (s,

6H, CH₃trans), 1.55 (s, 1.2H, CH₃cis). ³¹P{¹H} NMR (CD₂Cl₂at

233 K), cis product: −45.1 (d, P_X, J_PXPA= 388 Hz), 12.9 (dd, P_A,

J_PAPM= 4.9 Hz), 18.8 (t, P_M). trans product: 35.1 (br, t, P_X), 36.2

(d, P_A, J_PAPX= 23.1 Hz).

162.0 Hz, J_HPt= 797 Hz). ³¹P{¹H} NMR (121.5 MHz, CD₂Cl₂,

295 K): 2a-Me cis, 20.7 (P_B, J_PAPB= 372 Hz, J_PBPX= −21.9 Hz,

J_PBPt= 3079 Hz), 23.6 (P_X, J_PXPA= 18.2 Hz, J_PxPt= 2395 Hz), 29.9

(P_A, J_PAPt= 2750 Hz). 2a-Me trans, 22.9 (P_X, J_PXPA= 19.4 Hz, J_PxPt

= 2255 Hz), 25.3 (P_A, J_PAPt= 2863 Hz).

Anal. Calcd for C₅₆H₄₆P₃O₇NS₂F₆Pd: C 55.02; H 3.79.

Found: C 55.10; H 3.92.

DPEphos(2-(diphenylphosphino)pyridine)hydridoplatinum(^II)-

bis-triflate (Pt2b)

dppf(2-(Diphenylphosphino)pyridine)hydridoplatinum(^II)bis-

triflate (Pt2a)

This compound was prepared as described for Pt2a-Me using

200 mg (0.155 mmol) of Pt1b dissolved in 20 mL of CH₂Cl₂.

The crude oﬀ-white solid was washed with 10 mL of pentane

and re-crystallized from CH₂Cl₂/Et₂O to aﬀord pure Pt2b as a

white solid. Yield: 170 mg (0.131 mmol), 85%.

43 μL of diethylamine were added to a solution of 540 mg

(0.412 mmol) of Pt1a in 20 mL CH₂Cl₂. This solution was

introduced into a stainless steal autoclave under argon, and

purged 3 times with H₂, after which the autoclave was pressur-

ized to 4 bar of H₂and heated to 40 °C overnight. After redu-

cing the pressure to 1 bar and cooling to room temperature,

the reaction mixture was transferred to a Schlenk flask under

1 bar of H₂. The yellow brown solution was evaporated under

vacuum to obtain an amber sticky solid, which after thorough

washing with diethyl ether (5 × 10 mL) and drying under

vacuum aﬀorded a golden powder. The ammonium salt was

removed by crystallization from CH₂Cl₂/Et₂O.

¹H NMR (300 MHz, CD₂Cl₂, 295 K): 8.50 (d, 1H, Py-H⁶),

7.77–6.87 (m, 38H, Ar), 6.78 (t, 1H, Py, J = 7.3 Hz), 6.61 (t, 1H,

Py, J = 9.5 Hz), 6.07 (m, 1H, Py), −6.81 (s, 1H, hydride, J_HPA

=

13.0 Hz, J_HPB= 16.5 Hz, J_HPX= 163.1 Hz, J_HPt= 739 Hz). ³¹P{¹H}

NMR (121.5 MHz, CD₂Cl₂, 295 K): 16.4 (P_B, J_PAPB= 361 Hz,

J_PBPX= −10.5 Hz, J_PBPt= 2944 Hz); 16.5 (P_X, J_PXPA= 31.7 Hz,

J_PxPt= 2217 Hz); 23.4 (P_A, J_PAPt= 2905 Hz).

Anal. Calcd for C₅₅H₄₄P₃O₇NS₂F₆Pt: C 50.93; H 3.42. Found:

C 51.05; H 3.45.

100 mg (0.076 mmol) of this complex was dissolved in

10 mL of CH₂Cl₂and the solution was frozen in an acetone/

liquid nitrogen bath. 10.1 μL (17.16 mg, 0.114 mmol) of HOTf

were added using a microsyringe. The Schlenk flask was

Sixantphos(2-(diphenylphosphino)pyridine)hydridoplatinum(^II)-

bis-triflate (Pt2c)

shaken to dissolve the acid and the reaction mixture was This compound was prepared as described for Pt2a-Me using

slowly warmed to room temperature. After 1 h at room temp- 200 mg (0.148 mmol) of Pt1c. The crude oﬀ-white product was

erature, the solvent was evaporated in a vacuum. The oily re-crystallized from CH₂Cl₂/pentane to aﬀord pure Pt2c as a

orange product was washed several times with diethyl ether to white solid. Yield: 180 mg (0.133 mmol), 90%.

obtain a dark orange solid. Yield: 80 mg (0.061 mmol), 72%.

¹H NMR (300 MHz, CD₂Cl₂, 295 K): 15.1 (br, NH), 9.12 (m,

¹H NMR (300 MHz, CD₂Cl₂, 295 K): 15.1 (br, NH), 8.54 (br, 1 H, Py-H⁶), 8.50–6.75 (m, 39 H, Ar), 0.75 (s, 6H, CH₃), −7.53

1

1H, Py-H⁶), 8.35 (br, H), 7.84–7.07 (m, 32H, Ar), 4.87 (s, 2H, (s, 1H, hydride, J_HPA= 13.7 Hz, J_HPB= 14.7 Hz, J_HPX= 158 Hz,

6508 | Dalton Trans., 2013, 42, 6495–6512

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Dalton Transactions

Paper

J_HPt= 732 Hz). ³¹P{¹H} NMR (121.5 MHz, CD₂Cl₂, 295 K): 15.9 dppf(2-Diphenyphosphine)(pyridinium)palladium(^II)bis-

(P_X, J_PXPA= −12.7 Hz, J_PXPB= 24.3 Hz, J_PxPt= 2286 Hz), 20.1 triflate (Pd3a)

(P_B, J_PAPB= 370 Hz, J_PBPt= 2030 Hz), 23.8 (P_A, J_PAPt= 2922 Hz).

This compound was obtained during the crystallization of

¹⁹⁵Pt NMR (HMQC, 85.6 MHz, CD₂Cl₂, 273 K): −5090.

Pd2a from CH₂Cl₂/Et₂O at room temperature.

Anal. Calcd for C₅₇H₄₈P₃O₇NS₂F₆SiPt·CH₂Cl₂: C 48.94; H

3.34. Found: C 49.14; H 3.13.

¹H NMR (300 MHz, CD₂Cl₂, 295 K): 13.5 (br., 1 H, NH),

7.9–7.07 (m, 34H, Ar), 6.0 ppm (ddd, P–H, ¹J_HP= 370 Hz, ³J_HP

=

16.8, 10.2 Hz), 5.16 (s, 1H, Cp), 4.76 (s, 1H, Cp), 4.65 (s, 2H,

Cp), 3.39 (s, 1H, Cp), 4.33 (s, 1H, Cp), 4.15 (s, 1H, Cp), 3.82 (s,

1H, Cp). ³¹P{¹H} NMR (121.5 MHz, CD₂Cl₂, 295 K): −1.6 (dd,

P_X, J_PXPA= 354 Hz, J_PXPM= 34.0 Hz), 17.86 (dd, P_M, J_PMPX= 14.6

Hz), 26.71 (dd, P_A).

Xantphos(2-(diphenylphosphino)pyridine)hydridoplatinum(^II)-

bis-triflate (Pt2d)

This compound was prepared as described for Pt2a-Me using

250 mg (0.187 mmol) of Pt1d. The crude pale yellow product

was re-crystallized from CH₂Cl₂/pentane to aﬀord pure Pt2d as

a white solid. Yield: 230 mg (0.172 mmol), 92%.

dppf(2-Diphenyphosphine)(6-methyl-pyridinium)palladium(^II)-

bis-triflate (Pd3a-Me)

¹H NMR (300 MHz, CD₂Cl₂, 295 K): 15.4 (br, NH), 9.17 (m,

1 H, Py-H⁶), 8.50–6.90 (m, 39 H, Ar), 1.83 (s, 6H, CH₃), −8.41

(s, 1H, hydride, J_HPA= J_HPB= 14.4 Hz, J_HPX= 159 Hz, J_HPt= 714

A Fisher–Porter bottle equipped with a magnetic stirring bar

was charged with 95 mg (0.077 mmol) of Pd1a-Me and 10 mL

of CH₂Cl₂were added. The dark-brown solution was cooled in

an ice bath and the bottle was pressurized with 3 bar of H₂.

The solution became light brown almost immediately and

after 15 min, it turned bright red. The solution was kept under

pressure for an additional 15 min and then transferred into a

Schlenk tube. The solvent was evaporated in a vacuum, the

remaining solid was washed with pentane (3 × 5 mL) and

dried under vacuum. Yield = 71.4 mg (0.058 mmol), 75%.

¹H NMR (300 MHz, CD₂Cl₂, 193 K): 12.05 (br., 1H, NH),

Hz). ³¹P{¹H} NMR (121.5 MHz, CD₂Cl₂, 295 K): 15.8 (P_X, J_PXPA

=

16.3 Hz, J_PXPB= −19.6 Hz, J_PxPt= 2298 Hz), 17.7 (P_B, J_PAPB= 375

Hz, J_PBPt= 2937 Hz); 23.0 (P_A, J_PBPt= 3045 Hz). ¹⁹⁵Pt NMR

(HMQC, 64.19 MHz, CD₂Cl₂, 273 K): −5090. IR ν(Pt–H) =

2099 cm⁻¹

.

Anal. Calcd for C₅₈H₄₈P₃O₇NS₂F₆Pt: C 52.1; H 3.6. Found: C

51.72; H 3.50.

1

9.31–7.04 (m, 34H, Ar), 6.1 ppm (ddd, P–H, J_HP= 375 Hz,

³J_HP= 16.8, 10.2 Hz), 5.21 (s, 1H, Cp), 4.69 (s, 1H, Cp), 4.48 (s,

4H, Cp), 4.41 (s, 1H, Cp), 4.35 (s, 1H, Cp), 3.79 (s, 1H, Cp), 1.53

(s, 3H, CH₃). ³¹P{¹H} NMR (121.5 MHz, CD₂Cl₂, 193 K): −2.2

High pressure NMR experiments

In a typical experiment the high pressure sapphire NMR tube

was charged with the corresponding starting material (1a–d)

and 2 mL of CD₂Cl₂were added inside a glove box. The closed

tube was then taken out of the box, cooled in an ethanol/N₂

bath when necessary, and pressurized with H₂. The tube was

then transferred to the pre-cooled NMR probe at the desired

temperature. The reactions were followed in time by ¹H and

³¹P NMR spectroscopy.

(dd, P_X, J_PXPA= 350 Hz, J_PXPM= 36.5 Hz), 17.7 (dd, P_M, J_PMPX

=

12.1 Hz), 27.2 (dd, P_A). ¹³C {¹H} NMR (100.6 MHz, CD₂Cl₂,

233 K): 179.5 (dt, C2 Py, J_CPtrans= 125.5 Hz, J_CPcis= 12.6 Hz);

157.1 (d, C₆Py, J_CP= 4.7 Hz); 141.2 (CH, Py); 136.3 (d, CH,

J_CP= 11.5 Hz); 134.8, 135.5, 134.3, 132.9, 132.6 (CH, Ar); 132.9

(d, CH, J_CP= 52.4 Hz); 131.9, 131.7, 131.5, 131.3 (d, CH, J_CP

9.8 Hz); 130.7 (d, CH, J_CP= 9.7 Hz); 130.5, 130.3 (d, CH,

J_CP= 11.1 Hz); 129.9 (d, CH, J_CP= 11.1 Hz); 129.6 (d, CH, J_CP

=

dppf(2-(Diphenylphosphino)pyridine)hydridopalladium(^II)bis-

triflate (Pd2a)

9.2 Hz); 128.8, 128.6 (d, CH, J_CP= 11.5 Hz); 128.2, 125.7, 125.2,

124.6 (C_quat); 122.4 (CH); 121.0 (q, CF₃, J_CF= 320.1 Hz); 120.8

(C_quat, J_CP= 51.9 Hz); 78.6 (d, CH, Cp, J_CP= 22.2 Hz); 77.4 (d,

CH, Cp, J_CP= 11.2 Hz); 76.9 (d, CH, Cp, J_CP= 7.6 Hz); 76.3 (d,

68 mg (0.056 mmol) of Pd1a were dissolved in 20 mL of

CH₂Cl₂and the purple solution was transferred to an autoclave

under Ar. The autoclave was pressurized to 10 bar of H₂and

the solution was allowed to react during 65 hours. The reaction

mixture was transferred to a Schlenk tube using H₂pressure

and the solvent was evaporated with a stream of H₂. The result-

ing light brown solid was washed with pentane (3 × 5 mL.) and

finally dried under vacuum. Yield = 50 mg (0.041 mmol),

73.4%.

CH, Cp, J_CP= 4.2 Hz); 75.3 (d, CH, Cp); 74.3 (d, CH, Cp, J_CP

13.9 Hz); 74.0 (d, CH, Cp, J_CP= 5.9 Hz); 72.6 (d, CH, Cp, J_CP

5.4 Hz); 70.6 (d, C_I, Cp, J_CP= 59.5 Hz); 70.2 (d, C_I, Cp, J_CP

51.8 Hz); 20.1 (CH₃).

=

Anal. Calcd for C₅₄H₄₄P₃O₆NS₂F₆PdFe: C 52.38; H 3.74.

Found: C 52.35; H 3.59.

¹H NMR (300 MHz, CD₂Cl₂, 295 K): 15.1 (br., 1 H, NH),

X-ray structures of Pt1b, Pt1c, trans1d and Pt2a-Me

8.5–7.1 (m, 34H, Ar), 4.96 (s, 2H, Cp), 4.64 (s, 2H, Cp), 4.33 (s, Pertinent data on the crystal structures have been deposited

2H, Cp), 3.69 (s, 2H, Cp), −6.7 (dd, J_HPX= 171 Hz, J_HPA/PB= 16, with the CCDC (904331–904334). Pt1b crystallizes in space

hydride). ³¹P{¹H} NMR (121.5 MHz, CD₂Cl₂, 295 K): 22.63 (t, group P1 with three molecules of dichloromethane. X-ray data

ˉ

P_X, J_PXPA/B= 26.9 Hz), 32.22 (dd, P_A/B).

were collected on an Enraf-Nonius CAD4T diﬀractometer on a

Anal. Calcd for C₅₄H₄₆P₃O₆NS₂F₆PdFe: C 52.00; H 3.62. rotating anode (MoKα) at 150 K. One of the triflate anions was

Found: C 51.86; H 3.58.

refined with a disorder model. R₁= 0.0669, wR₂= 0.178, S =

Dalton Trans., 2013, 42, 6495–6512 | 6509

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Dalton Transactions

1.029. Pt1c crystallizes in space group C2/c with one dichloro- 13 (a) D. H. Lee, B. P. Patel, E. Clot, O. Eisenstein and

methane, distributed over two locations of which one di-

sordered. X-ray data were collected on a Stoe IPDS (MoKα)

system at 180 K. The H-atoms of CH₂Cl₂were not included in

the model and refinement. R₁= 0.0395, wR₂= 0.1015, S =

1.045. trans1d crystallizes in space group C2/c with half a H₂O

molecule. X-ray data were collected on a Nonius KappaCCD on

R. H. Crabtree, J. Chem. Soc., Chem. Commun., 1999, 297;

(b) K. Gruet, E. Clot, O. Eisenstein, D. H. Lee, B. Patel,

A. Macchioni and R. H. Crabtree, New J. Chem., 2003, 27,

80; (c) T. Liu, S. Chen, M. J. O’Hagan, M. Rakowski DuBois,

R. M. Bullock and D. L. DuBois, J. Am. Chem. Soc., 2012,

134, 6257.

a rotating anode (MoKα) at 150 K. R₁= 0.0224, wR₂= 0.0562, S 14 A. Llamazares, H. W. Schmalle and H. Berke, Organometal-

= 1.007. Pt2a-Me crystallizes in space group P2₁/c with one lics, 2001, 20, 5277.

CH₂Cl₂molecule of crystallization. X-ray data were collected on 15 N. V. Belkova, L. M. Epstein and E. S. Shubina, Eur. J. Inorg.

a Nonius KappaCCD on a rotating anode (MoKα) at 150 K.

Chem., 2010, 3555.

16 C. P. Lau, S. M. Ng, G. Jia and Z. Lin, Coord. Chem. Rev.,

2007, 251, 2223.

17 R. Custelcean and J. E. Jackson, Chem. Rev., 2001, 101,

Acknowledgements

1963.

The work described here was financially supported by the 18 R. H. Crabtree, P. E. Siegbahn, O. Eisenstein, A. L. Rheingold

Council for Chemical Sciences of the Netherlands Organiz-

ation for Scientific Research (CW-NWO).

and T. F. Koetzle, Acc. Chem. Res., 1996, 29, 384.

19 (a) J. A. Ayllon, C. Gervaux, S. Sabo-Etienne and

B. Chaudret, Organometallics, 1997, 16, 2000; (b) Y. Guari,

J. A. Ayllon, S. Sabo-Etienne and B. Chaudret, Inorg. Chem.,

1998, 37, 640.

20 (a) E. S. Shubina, N. V. Belkova, E. V. Bakhmutova,

E. V. Vorontsov, V. I. Bakhmutov, A. V. Ionidis,

C. Bianchini, L. Marvelli, M. Peruzzini and L. M. Epstein,

Inorg. Chim. Acta, 1998, 280, 302; (b) N. V. Belkova,

P. A. Dub, M. Baya and J. Houghton, Inorg. Chim. Acta,

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21 S. H. Park, A. J. Lough, G. P. A. Yap and R. H. Morris,

J. Organomet. Chem., 2000, 609, 110.

22 M. Kranenburg, Y. E. M. van der Burgt, P. C. J. Kamer,

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Heterolytic activation of dihydrogen by platinum and palladium complexes

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