Homo and heterometallic assemblies from lewis acidic and basic metallic precursors 3

3.2 Results and Discussion 3.2.1 Synthesis and Characterisation of the Ligand-bridged Complexes of Dppf and 4,4’-Bipyridine The triflato complex 2.16a readily reacted with neutral bid

Chapter Three

Preparation of Dinuclear Complexes from

Mononuclear Pincer [PCP] Pd(II) Complexes

Chapter Three

Preparation of Dinuclear Complexes from

Mononuclear Pincer [PCP] Pd(II) Complexes

3.1 Introduction

It was mentioned in Chapter One that the pincer [PCP] complexes may serve as useful precursors to dinuclear complexes (Section 1.2) The dinuclear

species were detected in the ESI-MS studies One of these species viz 2.18 was

isolated and characterised The results have been discussed in Chapter Two In this chapter, the results on the synthesis of dinuclear complexes from complex

2.16a and other mononuclear complexes will be presented The structures of the

dinuclear complexes in both solution and solid states will be discussed

3.2 Results and Discussion

3.2.1 Synthesis and Characterisation of the Ligand-bridged

Complexes of Dppf and 4,4’-Bipyridine

The triflato complex 2.16a readily reacted with neutral bidentate ligands to

give the cationic dinuclear ligand bridged complexes [{Pd(PCP)}2(µ-L)][OTf]2 (L

= dppf, 3.1a; 4,4’-bpy, 3.3b) (Scheme 3-1) Both complexes were characterised by

NMR, elemental analysis and single-crystal X–ray diffraction

Scheme 3-1: Preparation of the dinuclear complexes 3.1a and 3.1b from complex 2.16a.

The 1H NMR spectrum of complex 3.1a showed that the protons of the Cp

rings in the dppf ligand were upfield shifted (2.85 ppm and 3.00 ppm) compared

to free dppf ligand (4.00 ppm and 4.25 ppm) The Cp protons are chemically shielded due to the ring current effect, imposed by the phenyl rings of both the bridging and chelating phosphines (Figure 3-1) The NMR data suggest close proximity of the phenyl rings and the Cp rings

H

H

α

α α

α

β β

β β

compared to 44.8 ppm in 2.16a The downfield 31P signals suggests deshielding of

Ppincer upon replacement of the triflato ligand by dppf and 4,4’-bipyridine

The 31P{1H} spectrum of complex 3.1a showed two signals, consistent

with the chemically inequivalent phosphines that are strongly coupled to each

other [δP(dppf) = 6.6 ppm (t); δP(pincer) = 55.1 ppm (d)] (Figure 3-2) The 2 J P-P is 44

Hz This value is comparable to that of [Pd(Me)(PMe3)3][hfac] (hfac = hexafluoroacetylacetonate) (2 J P-P = 43 Hz).127 In comparison to the known dppf-bridged complexes [{(dppf)PdCl}2(µ-dppf)][ClO4]2 (δP(dppf) = 26.8 ppm)128 and [Pd2(C2,N-dmpa)2(µ-dppf)Cl2] (dmpa =N,N–dimethyl-1-phenethylamine) (δP(dppf)

= 32.7 ppm),129 the observed chemical shift of the bridging dppf appears to be more shielded A singlet peak 49.8 ppm was observed in the 31P{1H} NMR

spectrum of complex 3.1b is consistent with the presence of chemically equivalent phosphines in complex 3.1b

0 10

20 30

40 50

Moreover, the signal of the metallated carbon was observed in the 13C{1H}

NMR spectrum of complex 3.1a at 79.3 ppm (dt) It is strongly coupled to the

dppf ligand at the trans position (2J C-P(dppf) = 28 Hz) and is weakly coupled to the phosphines of the pincer ligand (2J C-P( pincer) = 3 Hz) cis to it The signal of the

metallated carbon of complex 3.1b was observed at 60.2 ppm as a broad singlet

peak

A 31P{1H} NMR study suggested that complexes 3.1a and 3.1b are stable

in non-coordinating solvent (CDCl3) for two weeks However, the complexes underwent dissociation in CD3CN The 31P{1H} NMR spectrum of complex 3.1a

recorded in CD3CN gave a singlet at 51.3 ppm together with two small peaks at 49.5 ppm (s) and 55.1 ppm (s) The singlet at 51.3 ppm were assigned to the

CD3CN-coordinated species [Pd(CD3CN)(PCP)][OTf] 3.2, this assignment was

confirmed by comparing the 31P{1H} NMR spectrum of complex 2.16a recorded

in CD3CN Observation of a clean 31P{1H} NMR spectrum of complex 2.16a in

CD3CN suggested a facile substitution of the triflato ligand by CD3CN The small peaks of the spectrum have not been identified A signal that is close the free dppf

ligand (δP = -18.0 ppm) was also observed in the NMR spectrum of complex 3.1a,

possibly due to the dangling phosphine of the monocationic species

[Pd(PCP)(dppf)][OTf] 3.3 (Figure 3-3) This species was observed in the earlier

ESI-MS study (See Section 2.2.2)

The CH3CN-coordinated complexes of Pd(II) have been widely used as precursors to Pd(II) phosphine complexes.119 The reverse ligand displacement of a

These factors include the lack of chelating effect from the bridging dppf ligand

that is important in stabilising complex 3.1a, the high trans-effect of the

metallated carbon causing the dppf ligand to become labile, and the steric crowding of the phenyl rings of both dppf and pincer moiety that helps in pushing

the dppf ligand out from the pincer moiety The trans-effect can be associated with the trans-influence, as suggested by the NMR and crystallographic data (an

upfield 31P signal of the bridging dppf ligand and a long Pd-C bond) The steric crowding between the dppf ligand and the pincer moiety are indicated by the upfield shift of the Cp protons and the highly distorted square-planar coordination geometry around the Pd(II) centre

-20 -10

0 10

20 30

40 50

(ppm)

Figure 3-3: 31P{1H} NMR spectrum of complex 3.1a in CD3 CN (* = unknown; P a refers

to P dppf , P b refers to P pincer )

Pd PPh2PPh2 Fe

PPh2

Ph2P

PPh2

PPh2

NCCD3

3.2

In the case of complex 3.1b,the above mentioned singlet peak at 51.3 ppm was observed as the main peak together with two small peaks at 49.5 ppm (s) and 55.5 ppm (s) when CD3CN was used as solvent The 31P{1H} NMR spectrum suggested a facile substitution of the 4,4’-bipyridine ligand by an CD3CN ligand This result is however contradictory to the previous ESI-MS study Although the molecular ion was not observed in the ESI mass spectrum, the monocationic species [Pd(PCP)(4,4’-bpy)]+ was detected (See Section 2.2.2)

3.2.2 Crystal Structures of Complexes 3.1a and 3.2b

The crystal structure of complex 3.1a shows that the pincer moieties are

bridged by a dppf ligand (Figure 3-4) The ferrocenyl moiety is surrounded by the phenyl rings of the P atoms in both dppf and the pincer ligands This observation

is in agreement with the upfield Cp protons shift in the 1H NMR spectrum of

complex 3.1a Lengthening of the Pd-C bond was observed from 2.071(2) Å in 2.16a to an average of 2.126(7) Å in 3.1a upon replacement of the triflato ligand

by dppf indicating weakening of that bond The Pd-P bonds trans to the metallated carbon [average bond length = 2.369(2) Å] are longer than those cis to the metallated carbon [average bond lengths = 2.313(2) Å] This is due to the trans-

influence of the metallated carbon and the steric crowding between the pincer

moieties and the dppf ligand The trans-influence and the steric effect also cause

distortion of the coordination geometry around the two Pd(II) atoms This can be seen from the C-Pd-Pdppf [C(1)– Pd(1)– P(5) = 166.9(2)°; C(6)-Pd(2)-P(6) = 167.9(2)°] and the P-Pd-P angles [P(1)-Pd(1)-P(2) = 159.7(7)° ; P(3) –Pd(2) –

precursor complex 3.1a (L = OOTf) [C – Pd – O = 171.7(9)°; Ppincer-Pd- Ppincer = 166.2(2)°]

Figure 3-4: An ORTEP plot of complex 3.1a with 50% thermal ellipsoids The OTf

-anions are omitted for clarity Selected bond lengths (Å) and angles (º): Pd(1)-P(1) = 2.322(2), Pd(1)-P(2) = 2.317(2), Pd(2)-P(3) = 2.313(2), Pd(2)-P(4) = 2.303(2), Pd(1)-P(5)

= 2.376(2), P(2)-P(6) = 2.363(2), Pd(1)-C(1) = 2.122(6), Pd(2)-C(6) = 2.130(7), C(1)-C(2)

= 1.507(1), C(4) = 1.526(9), C(6)-C(7) = 1.478(1), C(6)-C(9) = 1.533(1); Pd(1)-P(5) = 166.9(2), C(6)-Pd(2)-P(6) = 167.9(2), P(1)-Pd(1)-P(2) = 159.7(7), P(3)- Pd(2)-P(4) = 159.3(8), P(1)-Pd(1)-C(1) = 81.2(2), P(2)-Pd(1)-C(1) = 80.9(2), P(3)-Pd(2)- C(6) = 80.7(3), P(4)-Pd(2)-C(6) = 81.7(2), P(1)-Pd(1)-P(5) = 99.1(6), P(2)-Pd(1)-P(5) = 100.5(7), P(3)-Pd(2)-P(6) = 99.2(7), P(4)-Pd(2)-P(6) = 100.3(8), Pd(1)-C(1)-C(2) = 115.3(5), Pd(1)-C(1)-C(4) = 116.8(5), Pd(2)-C(6)-C(7) = 115.7(6), Pd(2)-C(6)-C(9) = 113.9(5), Pd(1)-P(5)-C(11) = 116.5(2), Pd(2)-P(6)-C(16) = 118.3(2)

C(1)-The crystal structure of complex 3.1b shows a dinuclear framework with a

bipyridyl ligand bridging across the two square-planar Pd(II) pincers, as shown in Figure 3-5 The angle between the coordination plane and the pyridyl ring deviates significantly from 90º [P(1)-Pd(1)-P(2)-C(1)-N(1)/N(1)-C(6)-C(7)-C(8)-C(9)-C(10) = 80.4º] This value is smaller than those angles observed for the related square-planar d8 complexes [(Pt(pip2NCN))2(μ-L)]2+ (pip2NCN- = 1,3-

bis(piperidylmethyl)phenyl; L = 4,4’-bipyridine, pyrazine or

trans-1,2-bis(4-pyridyl)ethylene)where the dihedral angles between the Pt(II) coordination planes and the pyridyl rings are in the range of 83.3º to 89.9˚.12b,12c

The Pd-C bond length in 3.1b [Pd(1)-C(1) = 2.088(4) Å] is intermediate between that of the complex 2.1 [Pd(1)-C(1) = 2.111(7) Å] and 2.16a [Pd-C = 2.071(2) Å] The Pd-N bond [Pd(1)-N(1) = 2.187(4) Å] in complex 3.1b is longer

than those Pd-N(pyridyl) bonds in the metallomacrocyclic compounds

[{Pd(dppm)(μ-L)}2][OTf]4 (dppm = bis(diphenylphosphino)methane; L = NC5H43-CH2NHCOCONHCH2-3-C5H4N or N,N’- bis(pyridine-4-yl)-pyridine-2,6-

-dicarboxamide) and the polymeric complex [{Pd(dppp)(μ-L)}x][OTf]2x (dppp = 1,3-bis(diphenylphosphino)propane ; L = N,N’- bis(pyridine-4-yl)-isophthalamide) where their Pd-N bond lengths fall in between 2.095(4) Å and 2.114(5) Å.130 The

lengthening of Pd-N bond suggests a high trans-influence of the metallated carbon

in 3.1b

Figure 3-5: An ORTEP plot of complex 3.1b with 50% thermal ellipsoids The OTf

-anions and (CH 3 ) 2 CO molecule are omitted for clarity Selected bond lengths (Å) and angles (º): Pd(1)-P(1) = 2.324(1), Pd(1)-P(2) = 2.317(1), Pd(1)-N(1) = 2.187(4), Pd(1)- C(1) = 2.088(4), C(1)-C(2) = 1.499(7), C(1)-C(4) = 1.492(7), C(2)-C(3) = 1.453(8), C(4)- C(5) = 1.486(8), C(8)-C(8)#1 = 1.511(8); P(1)-Pd(1)-P(2) = 164,4(5), P(1)-Pd(1)-N(1) = 99.0(1), P(2)-Pd(1)-N(1) = 96.4(1), P(1)-Pd(1)-C(1) = 82.5(1), P(2)-Pd(1)-C(1) = 82.0(1), N(1)-Pd(1)-C(1) = 176.8(2), Pd(1)-C(1)-C(2) = 113.8(3), Pd(1)-C(1)-C(4) = 113.2(3), C(2)-C(1)-C(4) = 115.2(5) Dihedral angle (º): P(1)-Pd(1)-P(2)-C(1)-N(1)/N(1)-C(6)- C(7)-C(8)-C(9)-C(10) = 80.4

3.2.3 Synthesis and Characterisation of the Pincer [PCP] Pd(II)

gave complicated mass spectra suggesting that fragmentation readily occurs (Section 2.2.2) This approach was therefore not pursued further An alternative to this method was to prepare a pincer metalloligand and study its coordination chemistry with a series of cationic metal complexes Despite many reported works

on 4-pyridinethiolato complexes, the pincer Pd(II) and Pt(II) pyridinethiolato complexes have been reported only recently.134 The metalloligand [Pd(4-

Spy)(PCP)] 3.4 was prepared by a standard ligand replacement reaction135 of

complex 2.1 with 4-pyridinethiolate ligand The thiolato ligand was generated

in-situ from 4-pyridinethiol and KOH (Scheme 3-2)

KCl

CH2Cl2 /(CH3)2CO / CH3OH

Pd S PPh2

PPh2

N

RT (1:1:1 v/v)

3.4 2.1

Scheme 3-2: Synthesis of complex 3.4 from complex 2.1

The 13C{1H} and 31P{1H} NMR spectrum of complex 3.4 suggested that

the metallated carbon and the phosphorus atoms of the pincer ligand are

deshielded (δC = 65.3 ppm and δP = 46.4 ppm) as compare to complex 2.1 [δC =

60.8 ppm; δP = 41.8 ppm] as a result of replacement of chloro ligand by the soft

4-pyridinethiolato ligand The ESI mass spectrum of complex 3.4 gave a peak at m/z

= 656 which could be assigned to the protonated ion [Pd(PCP)(4-SpyH)]+ [3.4 +

H+]+ (calculated m/z = 656)

The structure of complex 3.4 was confirmed by a single-crystal X-ray analysis of the complex (Figure 3-6) Complex 3.4 represents a rare Pd(II) pincer

with a 4-pyridinethiolato ligand trans to the metallated carbon The

Pd(I)-S(1)-C(6) angle is 112.7(1)˚ The Pd(1)-C(1) bond length [2.117(3)Å] and the Pd(1)-S(1) angle [177.4(8)˚] are comparable to the Pd-C bond and the C-Pd-Cl

C(1)-angle observed for complex 2.1 [Pd(1)-C(1) = 2.111(7) Å; C(1)-Pd(1)-Cl(1) =

177.3(2)˚] However, it is notable that the P(1)-Pd(1)-P(2) angle [155.3(3)˚] appears to be the smallest among the complexes observed

Figure 3-6: An ORTEP plot of complex 3.4 · 0.5 (CH 3 ) 2 CO with 50% thermal ellipsoids

The (CH 3 ) 2 CO molecule is omitted for clarity Selected bond lengths (Å) and angles (º): Pd(1)-P(1) = 2.301(7), Pd(1)-P(2) = 2.282(8), Pd(1)-S(1) = 2.403(8), Pd(1)-C(1) = 2.117(3), C(1)-C(2) = 1.528(4), C(1)-C(4) = 1.522(4); P(1)-Pd(1)-P(2) = 155.3(3), P(1)- Pd(1)-S(1) = 96.3(3), P(2)-Pd(1)-S(1) = 100.3(3), P(1)-Pd(1)-C(1) = 82.3(8), P(2)-Pd(1)- C(1) = 81.7(8), S(1)-Pd(1)-C(1) = 177.4(8), Pd(1)-C(1)-C(2) = 114.0(2), Pd(1)-C(1)-C(4)

= 115.8(2), C(2)-C(1)-C(4) = 111.1(2), C(6)-S(1)-Pd(1) = 112.7(1)

Attempted reactions of complex 3.4 with metal salts like Ni(acac)2, Mn(acac)2 · 3H2O and copper acetate hydrate were carried out with the aim of obtaining Pd(II)/M [M = Ni(II), Mn(II), Cu(II)] heterometallic complexes These metal salts were chosen because the metal centres have a preferred octahedral coordination geometry that might overcome the steric crowding caused by

complex 3.4 However, these reactions formed insoluble precipitates, which

precluded their characterisation

It has been reported that the N-bonded Ag(I) pyridinethiolato complexes136and the polymer137 containing a ferrocenyl backbone can be conveniently prepared

by a suitable ligand and Ag(I) salts Using a similar approach, the reactivity of

complex 3.4 towards Ag(I) salts was investigated Reaction of complex 3.4 with

one equivalent of AgNO3 gave a suspension which was filtered to give both a white precipitate and a yellow solution The white precipitate was presumably the Ag(I) pyridinethiolato polymer (Scheme 3-3) A similar polymer obtained from the analogous reaction of 2-pyridinethiolato ligand with AgBF4 has been reported.138 The 31P{1H} NMR spectrum of the yellow solution recorded in CDCl3 suggested that the mixture contains complex 2.16b as major product

together with an unidentified small peak at 48.6 ppm A similar reaction of

complex 3.4 with half an equivalent amount of AgPF6 gave the bridged dinuclear homometallic Pd(II) complex [{Pd(PCP)}2(μ-4-Spy)][PF6] 3.5

4-pyridinethiolato-in low yield (18 %) and a white solid as observed earlier The pure product was obtained by fractional crystallisation from CH2Cl2/Diethyl ether It is important to note that use of excess AgPF6 resulted in the disappearance of signal of complex

PF6

Scheme 3-3: Reactions of complex 3.4 with AgNO3 and AgPF 6

Complex 3.5 is presumably formed from a reaction between 3.4 and 2.17a

To support this, the complex was synthesised independently from 3.4 and

[Pd(H2O)(PCP)][PF6] 2.17b Complex 2.17b can be prepared in an analogous

fashion to its BF4- analogue The reaction pathway is shown in Scheme 3-4

Isolation of complex 3.5 illustrates the potential of 3.4 as a metalloligand that

carries both a pincer and a 4-pyridinethiolato ligand

PPh 2

N Pd

Ph 2

P

Ph 2 P

Pd PPh2

PPh2

OH2 PF6

Pd PPh2

Scheme 3-4: A designed synthesis of complex 3.5

The NMR spectra [1H, 31P{1H} and 13C{1H}] of complex 3.5 gave two sets

of signals, suggesting that the two pincer Pd(II) moieties are chemically inequivalent The signals that correspond to the two protons attached to the metallated carbon were easily distinguishable in the 1H NMR spectrum of

complex 3.5, and were observed as two unresolved triplet peaks at 3.17 and 3.32

ppm (2 J H-H = 12 Hz) The signals due to the metallated carbons were observed at 59.5 ppm and 66.3 ppm as two broad peaks in the 13C{1H} NMR spectrum, corresponding to the N-bonded and S-bonded Pd(II) moiety respectively The signals of the methylene carbons could also be easily assigned (see experimental

31 1

moiety A signal was observed at 48.1 ppm (s) which can be assigned to the two P atoms of the N-bonded pincer Pd(II) moiety The 31P{1H} NMR spectrum of

complex 3.5 is shown in Figure 3-7 As observed for complexes 2.19, complex 3.5

is stable in CDCl3 but slowly undergoes decomposition in CD3CN

43.5 44.0 44.5 45.0 45.5 46.0 46.5 47.0 47.5 48.0 48.5 49.0

The ESI-MS analysis of complex 3.5 provided further support to its

formation The molecular ion [{Pd(PCP)}2(μ-4-Spy)]+ [3.5 – PF6-]+ peak was

observed at m/z = 1200 (calculated m/z = 1202) The related fragment ions [2.1

-Cl-]+ and [3.4 + H+]+ have also been detected (Figure 3-8)

Pb

Pa