Synthesis and characterisation of bis(ferrocenylethynyl) complexes of platinum (II) A re-investigation of their electrochemical behaviour

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ELSEVIER

Inorganic Chemistry Communications 1 (1998) 239-245

Synthesis and characterisation of bis ( ferrocenylethynyl) complexes of platinum ( II) A re-investigation of their electrochemical behaviour Domenico Osella aT*,Roberto Gobetto a, Carlo Nervi a, Maw-o Ravera a, Rosaria D’Amato b, Maria Vittoria Russo b,* aDipartimento di Chimica IFM, Universitri di Torino, Vi0 Pietro Giurin 7, 1012.5 Turin, Italy b Diparrimento di Chimica, Universitir ‘Lo Sapienza’, P. le Aldo More 5, 00185 Rome, Italy

Received I2 June 1998

Abstract We describe herein the synthesis and the electrochemical behavionr of a series of his( ferrocenylethynyl) complexes of platinum( II). In all complexes, the electronic interaction between the redox-active iron cores of the ferrocenyl termini is small, indicating a moderate electronic 6 1998Elsevier Science S.A. All rights reserved. delocalisation over the bis( cyclopentadienyl-acetylide)Pt chain. Keywords: Electron transfer; Platinum complexes;

Ferrocene complexes

1. Introduction

2. Results and discussion

Acetylide complexes of transition metals have recently received a good deal of attention owing to their peculiar new properties. These a-bonded metal-acetylides are precursors of rod-like polymers [ 11, which are promising low dimensional conducting (LDC ) materials for applications in molecular electronics and show liquid-crystal and second and third order optical non-linear (NLO) properties [ 21. The acetylene spacers can act as electronic bridges between adjacent transition metal centres [ 31. Whatever intra-molecular conductivity or molecular hyperpoltizability is desired, one necessary requirement is a high electronic delocalisation through the organic chain. In principle, electrochemical techniques, in solution, are able to detect moderate to strong electronic interactions between the redox centres provided they undergo fully reversible processes. This view has been extensively developed by Robinson and co-workers on tricobalt clusters [ 41, and by Geiger and co-workers on arene-chromiumderivatives [5]. In this paper we report the synthesis and characterisation of a series of pt(II) acetylides with particular attention to the electrochemical behaviour of bis( ferrocenylethynyl) complexes.

2.1. Characterisation Compounds 1-14 (Fig. 1) were characterised by elemental analysis, IR, ‘H and 13CNMR spectroscopy and mass spectrometry (see Section 4). These compounds, which have fairly high molecular weights and low polarity, are much more effectively characterised by the desorption chemical ionisation (DCI) technique [ 61 (reagent gas isobutane) than either electron impact (EI, ionising energy = 70 eV) or fast atom bombardment (FAB, glycerol matrix) ionisation methods. Only the DC1 technique provides excellent sensitivity

1

L= PPh,, R= R’= ph

2

L= PPb.

4

L= PPh,. R= Ph, R’= Fc

R= R’= Fc

7

L= P(ptdylh.

R= R’= Ph

8

L= P(p-tdylh.

R= R= Fc

9

L= PMephz. R= R’= Ph

IO L= Pm,

5 X=cI 6 X=H

3

L= PPh3. R= Fc

13 L-L= 14 L-L=

R= R’= Fc

11 L= PBus. R= R’= Ph

* Corresponding authors. Osella: Tel.: + 39-01 l-6707507/8; 01 l-670 7855; E-mail: [email protected] 1387-7003/98/$ - see front matter 0 PUS1387-7003(98)00064-l

Fax: + 39-

12 L- PBus. R= W- Fc

1998 Eisevier Science S.A. All rights reserved.

Fig. 1. Sketch of the stn~ctures of complexes

l-14.

dppa. R= Ph dppe. R= Fc

240

D. Osella et al. /Inorganic Chemistry Communications 1 (1998) 239-245

for the molecular ions of the complexes under investigation. Fig. 2 shows the DC1 mass spectrum of truns-Pt(PPh,),(-C=C-Fc), (2) in the m/z range 500-1500, as an example. The quasi-molecular ion [M + H] + exhibits the most abundant peak-pattern in this range (centred at m/z 1138, based on 19%). Two other peaks are present in the spectrum, the former at higher m/z value (namely 1194) corresponds to the adduct [M+ ( C4H9) ] + , the latter to the fragment ion [Pt(PPh,),(-C=C-Fc)] +. Noteworthy, the most intense peak in the overall spectrum corresponds to the [ PPh3 + H] + ion (mlz = 263), as expected on the basis of the high basic@ of the phosphine. The solution NMR parameters of the fourteen compounds examined are listed in Table 1. Some observations are worth mentioning. The 3’P spectra of the platinum complexes show satellites in the relative ratio 1:3.9: 1 with respect to the central peak. The magnitude of ‘J(‘95Pt3’P) is known to be very sensitive to the nature of the ligands in square planar platinum( II) complexes and can, thus, act as a useful probe into the geometry of any isomers present [ 71. The ‘J( ‘95Pt3’P) values obtained for the compounds examined here are in agreement with the values found in other square planar platinum complexes with the truns-isomers showing larger platinum-phosphorus coupling constants than the cis-analogues (as previously reported by several authors [ g-101) . For compounds soluble enough to obtain 13C NMR in natural abundance, differentiation between cis- and trunscomplexes is possible by observing the multiplicity of the resonances of the acetylenic carbons. In cis-isomers (e.g. 3, 13 and 14) the resonance attributed to the a-carbon (directly linked to the Pt centre) appears as a doublet of doublets, due to a large truns-2Jw of about 150 Hz and a small cis-‘.I, of about 15 Hz. The resonance assigned to the P-carbon appears as a doublet having a trurw3J, of about 35 Hz while the cis-

3&c is negligible. In contrast, in rruns-complexes (e.g. 712) the resonance attributed to the a-carbon appears as a triplet, with a cis-*Jpc of about 1.5Hz, while the resonance of the P-carbon appears as a singlet. A similar relationship has been previously reported [ 111. The low solubility of the truns-Pt( PPh,) ,(-C=C-Fc) 2 isomer (2) meant that a 3’P solid state NMR investigation had to be carried out. A similar investigation was undertaken for cis-Pt( PPh3)*( -C=C-Fc) 2 (3) for the purpose of comparison. In a high resolution experiment at high magnetic field strength, the major differences on going from solution to solid state concern the greater number of resonances which are usually observed in the latter. In effect, for two nuclei to be equivalent in a solid sample requires a symmetrical environment within the molecule and also for the overall crystal frame. Solid-state inequivalences arise when crystal packing effects cause the site symmetry to be lower than the molecular symmetry or when the unit cell contains different molecules or, finally, when the same molecule crystallises in different unit cells. The 31Psolid state NMR spectra of 2 and 3 recorded in CPMAS condition are reported in Fig. 3. The truns- isomer (2) shows in the isotropic region a single peak centred at 23.3 ppm flanked by the usual ‘95Pt satellites [ ‘.I( ‘95Pt, 3’P = 2679.4 Hz) 1. On the contrary, the cis- isomer (3) exhibits two distinct peaks of the same intensity (each flanked by “‘Pt satellites) at 20.06 [ ‘.I( ‘95Pt?‘P = 2368.1 Hz) ] and 12.6 [ ‘.I( ‘95Pt,3’P= 2142.3 Hz) ] ppm, respectively, related to the presence of two crystallographically independentphosphorus environments in the unit cell. 2.2. Electrochemical behuviour In a previous paper we reported the electrochemical behaviour of the poorly soluble fruns- [Pt( PPh,),( -C=C-Fc) *] I

Fig. 2. DC1 mass spectrum of trans-Pt(PPh&(-C=C-Fc), (toe).

(2) in the m/z range 500-1500 along with experimental and simulated patterns of [M+H] + ion

2.9 ( ‘JRp= 2383)

41.7 (‘JRp=2280)

40.9 (‘Jptp=2281)

12

13

14

8.04-7.47 (m,20H,Ph);4.17,3.94 (m, 4H, -CH,-CH,-)

(s,Cp);2.44

2.14, 1.62, 1.54 (m, 36H,

(t,8H,C,H4,3Jmr=2);3.91

4.14 (s, lOH, Cp); 4.22,4.05 (t, 8H, &I&, 3Jrm=2); -CH*- CH,-CH,-); 0.99 (t, 18H, Me; ‘JHH = 7) 8.02-7.05 (m, 30H, Ph) ; 2.45 (m, 4H, -CH&H,-)

(m. Ph); 69.2 (s, Cp); 72.1.70.1,

66.3 (s, c~H,)

(m, Ph); 111.3 (s, C,(p));

108.3 (t, c,(o),

hLack of solubility in common organic solvents prevented measurement

2Jpc=32);

28.2

26.4,24.4,

2~pc= 15); 69.1

2~pc= 15); 14.8

103.8 (s> C,,(P)); 102.9 (t. C,((Y), ‘Jpc= 15); 69.3 (s, Cp); 71.6,70.1,66.8 (s, GH,); 26.4,24.3,23.6 (vt, CH,); 13.8 (s, Me) 133.5-124.9 Cm, Ph); 112.1 (d, C,(p), ‘~,=35); 104.1 (dd, c,(o), *J,(frans-) = 147, ‘&(cis-) = 15); 28.4 (m, CH,) 133.5-128.6 (m, Ph); 113.2 (d, C,(p), 3Jpc=33); 105.3 (dd, c,(o), ‘Jpc(rrobr-) = 145, *Jpc(cir-) = 13); 69.3 (s, Cp); 71.3, 70.6.66.7 (s, C&); (m, CHa)

133.6127.9 (m, Ph); 106.6 (s, C,(B)); 102.9 (t, c,(o), (s, Cp); 72.4,70.0,66.6 (s, Csh); 15.0 (t. Me, l&=21) 130.1-124.8 (m, Ph); 108.9 (s, C,(p)); 108.1 (t, c,(o), 23.9 (vt, CH,); 13.8 (s, Me)

(t, Me, ‘Jpc=21)

133.3-124.7

139.9-124.2 (m, C&and Ph); 112.7, 111.6 (s, C,); 21.3 (s, Me) 139.7-128.2 (m, Ph); 108.0-105.7 (s, C,,); 68.9 (s, cp); 72.9,69.7,66.1 (s, &HI); 21.2 (s, Me)

135.2-127.9

h

b

b

b

b

13C( ‘HI

a In CDCI,, chemical shifts (6) in ppm referred to 85% H3P04 for 3’P and to TMS for ‘H and 13C nuclei; coupling constants (J) in Hz ( f 1.O Hz), of the relative NMR spectrum. vt = virtual triplet as a result of an AA’X system (A = A’ = 3’P, X = H).

3.7 ( ‘JRp = 2362)

11

(‘J,,=2565)

-1.8

- 1.0 (‘J,,=2531)

9

10

15.5 ( ‘JRp = 2625) 14.7 ( ‘Jptp= 2658)

7.80-7.25 (m, 20H, Ph); 3.78 (s, lOH,Cp); 3.73.3.40 (t, 8H, C,I&, 3J,,=2); 2.27 (dt, 6H,Me,2JpH=4,3JptH=33) 7.29-7.11 (m, lOH,Ph); 2.17, 1.63, 1.43 (m, 36H,-CH,-CH,-CH,-);0.93 (t, 18H, Me, ‘Juu=7)

7.50-7.17 (m, 30H, Ph); 3.94 (s, IOH, Cp); 3.84,3.40 (t. 8H, CsH,, 3Jnu=2) 7.83-6.38 (m, 35H, Ph); 3.71 (s, 5H, Cp); 3.63,3.40 (t, 4H, C,H,,3J,,=2) 7.81-7.23 (m, 3OH, Ph); 3.68 (s, 5H, Cp); 3.72,3.25 (t. 4H, &I-L,, “J,,=2) 7.73-7.25 (m, 30H, Ph); 3.86 (s, 5H, Cp); 3.82, 3.67 (t, 4H, C,&, ‘J,,=2); -6.41 (dt, lH, hydride,‘J,n= 15, ‘JptH=650) 7.78-6.3 1 (m, 34H, CsH4 and Ph) ; 2.40 ( s, Me) 7.62-7.10 (m, 24H,C,HI); 3.65 (s,5H,Cp); 3.68, 3.33 (t, 8H, &HI, ‘J,,=2); 2.30 (s, Me) 7.73-6.64 (m, 3OH, Ph); 2.31 (dt, 6H, Me,ZJpH=3.6,3J,,=32.8)

16.8 19.6 22.1 28.7

(‘Jptp=2321) (‘Jptp= 2665) ( ‘JRp = 2684) ( ‘Jptp= 2932)

7.816.35 h

19.8 ( ‘Jptp=2650) b

(m, 40H, Ph)

‘H

“P( ‘H}

1-14 a

7 8

Compounds

Table 1 NMR data for complexes

f

N

D. Osella et al. /Inorganic Chemistry Communications1 (1998) 239-245

242

II 0

I

I

PI

0

-J-illh l2l

loo

a0

60

40

a

-im

0-20-4060-80

131

0

0 I c,

,~~‘,‘~‘1”‘,‘~~,‘~‘,~~~,~~‘,~~~,~~~,~~.,~~~

-IO0 Fr 3.5 A: statT NIZ spZra ofotra~~(~~~),(~~~~~)~ (2) (top) and cis- Pt( PPh,) a( -t&C-Fc) 2 (3) (bottom) recorded in CPMAS condition ( 109.4 MHz). Spinning sidebands are indicated with stars.

[ 12,131. In this investigation, we prepared a saturated solution of the above compound in THF by stirring the mixture overnight at room temperature and then filtering off the insoluble residue. In order to obtain more reliable CV responses we extended the synthesis and electrochemical investigation to a large series of bis( ferrocenylethynyl) bis( phosphine) R(R) derivatives, having various tertiary phosphines, in order to achieve better solubility. Surprisingly, none of these compounds showed either moderate or large electronic interactions. A i-e-investigation of the aforementioned THF solution showed that it actually consisted of a mixture of cis-[Pt(PPh,),(Cl),], cis-[Pt(PPh,),(-C=C-Fc),] (3) and truns- [ Pt( PPh3) 2( Cl) (-C=C-Fc) ] (5)) reagents and by-products in the synthesis of the desired rruns[ Pt( PPh,) 2(-C=C-Fc) *] (2) complex. In spite of their low abundance in the original mixture, these complexes had been selectively extracted by THF by virtue of their higher solubility [ 141. Unfortunately, the CV response of the above mixture gave two reduction processes for dierent Pt centres and two oxidation processes for different Fc moieties, and we erroneously interpreted this result as evidence of electronic coupling within the same compound [ 12,131. The CV

responses of pure 3, 5 and cis-[Pt(PPh,),(Cl)J [151 are consistent with this finding (Table 2). A pure sample of truns- [ Pt( PPh3) 2(-C=C-Fc) J, after several washings with THF, was found to be completely insoluble in common organic solvents; it was positively characterised by IR spectroscopy in KBr disks [ 161, solid-state 31P NMR spectroscopy and DC1 mass spectrometry. All the cyclic voltammetric (CV) responses of tetrahydrofuran (THF) solutions of soluble bis( ferrocenylethynyl) derivatives 3, 8, 10, 12 and 14, at a hanging mercury drop electrode (HMDE) , show a single, irreversible 2e reduction process in the cathodic sweep, together with some small peaks (due to reoxidation of electrogenerated fragments) in the reverse scan. The stoichiometry of such a process, assigned to the Pt(II) + Pt(0) reduction, has been confirmed by potential controlled coulometry at a mercury pool. This agrees with the usual electrochemical behaviour of Pt(I1) complexes [ 171. A broad peak system is observed in the anodic sweep (i,li,= 1.0, apparent AI!&= 100-120 mV). This response can be carefully interpreted as the result of two superimposed le peaks (attributed to the oxidations of the two ferrocenyl moieties) where a small difference in formal electrode potentials (AE” I 100 mV) is present. A computer-aided technique for peak resolution enhancement is the so called derivative neopolurogruphy [ 18,191, where the CV data are first semi-integrated (producing neopolurogrums) and then differentiated with respect to time (producing derivative neopolurogrums) . The result of such a data analysis for the anodic CV response of 10 produces two peak couples separated by about 80 mV (Fig. 4). Several experimental pulsed techniques, such as square wave voltammetry (SWV) and differential pulse voltammetry (DPV) , are able to difTable 2 Redox potentials (V vs. SCE) for complexes 1-14 in THF solution (0.2 M Bu&PF,) at a hanging mercury drop electrode (HMDE) Compounds

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Er (O/-2)

-2.19 c - 2.24 - 2.25 - 2.20 - 2.47 - 2.45 - 2.43 - 2.30 - 2.40 -2.71 - 2.80 - 2.26 - 2.37

a

Es, (+1/+2)s

Al?’ (ox) (mV)

E

E

E

+ 0.75 + 0.43 +0.49 + 0.43

+ 0.68

70

+ 0.40

+ 0.48

80

+ 0.42

+ 0.50

80

+0.51

+ 0.59

80

+ 0.45

+0.52

70

L (o/+1)

b

“Measured at 0.2 V s-’ scan rate. b From SWV at 30 Hz. ’Lack of solubility in common organic solvents prevented measurement of the relative electrochemical response.

D. Osella et al. /Inorganic

? .z H

4.000

-

2.000

-

0.000

-

-2.000

-

-4.000

-

-6.000

-

-6.000 650.0

I 750.0

Chemistry Communications I (1998) 239-245

I 650.0

I 550.0

450.0

350.0

E vs.

SCE (mV)

Fig. 4. CV response of a THF solution of 10 at an HMDE, scan rate 0.2 V s-

I

243

I

250.0

150.0

I 50.0

’(solid line) along with the corresponding

-50.0

derivative

neopolarogram

(dashed

line).

ferentiate between the two components of the oxidation process. The summit potentials (ES,) represent a good estimation of the formal electrode potentials (Z?‘) [ 201; for 10 the AESU is about 80 mV. The whole series of complexes shows similar electrochemical behaviour; obviously for compounds having acetylide ligands only the two-electron reduction of the platinum centre is observed (Table 2). Another indication of a weak electronic delocalisation comes from the similar hE”’ values obtained for rruns-bis ( ferrocenylethynyl) complexes (linear-shaped), namely 8, 10, 12, and cis-bis(fenocenylethynyl) complexes (L-shaped), namely 3 and 14. In principle, a higher degree of mixing between 7r orbital of acetylides and d orbitals of Pt( II) should be expected for the former class of compounds [ 2 I].

3. Conclusions The electronic interaction between the iron cores of the ferrocenyl moieties in complexes 3,8,10,12 and 14 is small. Several fulvene-like resonance formulae could be drawn in order to justify the electronic delocalisation (albeit small) experimentally found (Fig. 5). Two possible explanations can be proposed: (i) the -Pt( PR,) 2- interrupts the electronic communication within the organometallic spacers or (ii) the organometallic -C=C-Pt( PR3)&=Cchain does not directly link the Fe( II) redox termini, but their cyclopentadienyl rings. As far as hypothesis (i) is concerned, precedents in the literature are contrasting. The octahedral trunsC=C-Ru(dppm),-C=Callows a moderate (A,?? =220 mV) electronic communication between the two ferrocenyl termini [ 221. On the contrary, the planar trunsC=C-Pd(PBu,),-CrCwas described as an insulating block between the ReL, redox centres [ 231. As far as hypoth-

=

*cEcyi-czc-@J

8

F.a@

Ft?

80

0

II

Fig. 5. Schematic diagram mixed-valence complex.

of the bis( ferrocenylethynyl)platinum(II)

esis (ii) is considered, moderate electronic communications occur in Fc-CrC-Fc (AZ?”= 130 mV) [ 241 and Fc-C=CC=C-Fc ( AE”’= 100 mV) [ 251, while a very weak coupling (if any) was suggested in 1,Cbis( ferrocenylethynyl) benzene, ( [ Fc-C=C-C6H4-C=C-Fc] ) by Dixneuf and coworkers [26] and no interaction at all was found in 1,3,5tris( ferrocenylethynyl) benzene [ 271, where the Fe-Fe distances are comparable to those present in the complexes under investigation. We have planned the synthesis of the (dppm)CpFe-C=C-Pt( PR3),-CrC-FeCp( dppm) molecule, where the chain directly links the Fe redox termini. Whatever is the truth, only ‘pure’polyynediyl linkages guarantee remarkable electronic interactions, as in the cases of [ Cp*Fe( dppe) 12(@=C-C=C-) [ 28 ] and [ Cp*Re-

D. Osella et al. /Inorganic Chemistry Communications l(1998) 239-245

244

Table 3 Comprehensive data for the synthesis and characterisation of bis-acetylide Pt complexes Compounds

7 8 9 10 11

12 13 14

Reaction time (h)

18 6 0.5 1

Yield (%)

90 90 55 85

2

30

0.3 5 5

60 85 25

Crystalliiation solvent

toluene/EtOH CHaCla/EtOH CHClsIEtOH CHCl,/EtOH hexane/EtOH toluene/EtOH CHCl,/hexane C&/EtOH

UV” h (nm)

M.p. (“C)

213-215 226-228 193-194 > 250 (dec.) 84-85 104-105 216217 > 300 (dec.)

348 368,335 343 363,331 286,326 362 315 362

JRb VC-C (cm-‘)

2110 2112 2108 2120 2102 2100 2111 2115

Elemental analysis (talc. ) (%) C

H

70.20 (69.24) 62.75 (62.63) 63.54 (63.23) 59.32 (59.25) 59.45 (59.92) 56.40 (56.64) 63.20 (63.39) 58.77 (59.37)

5.64 (5.21) 4.84 (4.84) 4.78 (4.55) 4.38 (4.38) 7.35 (7.99) 7.20 (7.13) 4.09 (4.31) 4.18 (4.18)

a Spectra recorded in CHCls. b Spectra recorded in nujol mulls.

(NO) ( PPh,) ] 2( CL-C=C-C=C-) [291, reported by Lapinte’s and Gladysz’s groups respectively.

References t11 MS. Khan, A.K. Kakkar, N.J. Long, J. Lewis, P. Raithby, P. Nguyen,

4. Experimental The investigated complexes 1-14 were prepared according to previous published methods [ 301. Details of the reaction conditions and the characterisation performed with traditional techniques are reported in Table 3. IR and solution NMR spectra were recorded on a PerkinElmer 580 B and on a JEOL-EX 400 spectrometer, respectively. The high resolution 3’P cross-polarisation magic angle spinning ( CPMAS) NMR spectra were performed on a JEOL GSE 270 (6.34 T) spectrometer operating at 109.4 MHz. H3P0, was used as a reference ( 6= 0). Cylindrical 6 mm o.d. zirconia rotors with sample volume of 120 l.r,lwere employed with spinning speed in the range 4.0-5.0 KHz. DCI-MS spectra were recorded on a Finnigan-MAT 95Q instrument with magnetic and electrostatic analysers. Isobutane was used as the reagent gas at 0.5 mbar pressure. The ion source temperature was kept at 50°C the electron emission current at 0.2 mA, and the electron energy at 200 eV. Positive ion spectra were collected. Electrochemical apparatus and procedure have been previously reported [ 12,13 ] .

Acknowledgements We thank the National Research Council (CNR, Rome) for financial support (P.F. MSTA II), and Dr M. Vincenti (Dept. of Analytical Chemistry, University of Turin) for recording the DCI-MS spectra.

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[27] H. Fink, N.J. Long, A.J. Martin, G. Opromolla, A.J.P. White, D.J. Williams, P. Zanello, Organometallics I6 ( 1997) 2464. [28] N. Le Narvor, L. Toupet, C. Lapin@ J. Am. Chem. Sot. 117 (1995) 7129. [29] M. Brady, W. Weng, J.A. Gladysz, J. Chem. Sot., Chem. Commun. (1994) 2655. [ 301 M.V. Russo, A. Furlani. J. Organomet. Chem. 165 ( 1979) IO1.