13

Solid State Nuclear Magnetic Resonance, 1 (1992) 13-16

Elsevier Science Publishers B.V., Amsterdam

Solid state multinuclear NMR study of a-acetylide complexes of platinum, trans-[ClPt( P”Bu,) ,-CX-p-C,H,-GCPt(P”Bu,),Cl] and trans-[-Pt(P”Bu,),-CX-p-C,H,-GC-], Melinda J. Duer *, Muhammad S. Khan and Ashok K. Kakkar lJnil.ersity Chemical Laboratory, Lensfield Road, Cambridge CB2 IEW, UK

(Received 19 October 1991; accepted 25 October 1991)

Abstract

Solid state “C, slP and r’s Pt NMR has been employed to study the electronic and geometric structure of the dimeric and and transpolymeric a-acetylide complexes of platinum, trans-[ClPt(P”Bus),-C=C-p-C,H,-CX-Pt(P”Bu,)#] [-Pt(P”Bu,),-C=C-p-CeH,-C=C-I,,. The r9’Pt shielding tensor has been measured and analysed to reveal details about the electronic properties of the Pt-ligand bonds. Keywords: platinum; polymer; shielding tensor; phosphorous;

Introduction Transition metal containing o-acetylide complexes of the type truns-[-M(P”Bu,),-C-RC=C-], (M = Pt, Pd, Ni; R =p-C,H,; p-C&H,-C,H,-~1, first developed by Hagihara et al. [II, are of great importance due to their potential applications in the new materials industry. A number of recent reports on the synthesis [2-81 of such monomeric, oligomeric and polymeric complexes, trans-[-M(X”R3),-~C-R-~C-l, (M = Group 8-10 metal; R =p-C,H,, p-C,H,C,H,-p, nothing, -C=C-, -@C-p-C,H,-C=C-; X = P, As: R = Me, Et, Bu), and a study [5,8-121 of the properties of some of these complexes demonstrate the continued interest in this area. We wish to report a preliminary investigation of the solid state r3C, 3’P and ‘95Pt NMR spectra of the platinum metal containing polyyne polymer

* To whom correspondence

should be addressed.

acetylide

truns-[-Pt(P”Bu3)2-C&-p-C6H,-C~C-], and its precursor trans-[Cl(P”Bu,),Pt-C=C-pC,H,-GC-Pt(P”Bu,),Cl]. This study provides information about the nature of the Pt-ligand bonds and helps to elucidate the electronic and geometric structure of these complexes in the solid state.

Results and discussion Trans-[-Pt(P”Bu,),-C=C-p-C,H,-C=C-I, (1) and trans-[Cl(P”Bu,),Pt-C=C-p-C,H,-CzCPt(P”Bu&Cl] (2) employed in this study were prepared as desribed in ref. 7. ‘-‘C NMR The 13CNMR spectra of 1 and 2 were recorded using cross-polarisation (CP) and magic-angle spinning (MAS). Spectra were recorded on a Bruker MSL-400 spectrometer operating at

0X26-2040/92/$05.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved

M.J. Duer et al. /Solid

14

100.613 MHz. An optimal contact time of 8 ms and a recycle delay of 4 s were used. Both acetylide ‘jC resonances for the dimeric complex 2 show satellites due to ‘.iC-195Pt couplings (Fig. 1). The magnitudes of these couplings are 1470 Hz for the a-carbon (a> and 929 Hz for the P-carbon (b). Both of these values are very large for C-Pt coupling constants [13]. To the best of our knowledge, these are the largest observed to date for Pt(I1) complexes. In the case of the one-bond coupling constant between the (Ycarbon and platinum, this reflects the high degree of s-orbital character in the carbon orbital involved in the u-bond formation to the Pt(I1) ion. This is commensurate with the formal sp-hybridisation of the acetylide carbon. The value of 1470 Hz for this coupling constant is to be compared with those found [13] for various Pt(O)-CO linkages, which are often in excess of 2000 Hz; PBU,

Cl-

(1)

d

I

PBU,

b

P”lh,

FLcA&,_~,-c, I

State Nucl. Magn. Rrson. I (1992) 13-16

I

ppm Fig. 2. 13C CP/MAS

P”Bu,

from spectrum

TMS of the polymer

compound

1.

the carbon in these situations is again formally sp-hybridised. By contrast, 13C- 195Pt coupling constants for Pt-alkyl or Pt-alkene units are generally much lower, in the range of 200-800 Hz. In the spectrum of the polymeric material 1 (Fig. 2) no resonance is observed at all for the cu-acetylide carbon (a). Furthermore, the P-carbon (b) shows no couplings to ‘95Pt. The extreme sharpness of these resonances suggests that the polymer chains may be in motion. The molecular motion in these compounds is currently being investigated. “P NMR

ppm from TMS Fig. 1. “C CP/MAS spectrum of the dimeric precursor compound 2. The inset shows an expansion of the aromatic region. The satellites on the acetylide resonances due to ‘3C-‘ysPt coupling are indicated by an arrow; the relevant coupling constants are ‘Jpr_C = 1470 Hz and 2Jp,_c = 929 Hz.

The 31P MAS spectra of the polymer 1 and the precursor 2 are shown in Figs. 3 and 4. In both, a central broad resonance is observed at m 4.5 ppm relative to external 85% H,PO,. Satellites due to coupling of 31P to rs5Pt are also seen, yielding approximate values for the 31P-195Pt coupling constants of 2450 Hz for 1 and 2440 Hz for 2,

M.J. Duer et al. /Solid

State Nucl. Magn. Resort. 1 (1992) 13-16

a) *

I

b) Ppm from 85% H3P04 Fig. 3. “P MAS spectrum of the dimeric precursor compound 2. Satellites due to coupling to 195Pt yield an approximate value of 2454 Hz for the 31P-‘95Pt coupling constant. The broadness of the resonance is probably due to disorder of the phosphine groups in the crystal.

respectively. These values are typical of tram geometries at the metal centre [131. The tram effect ensures that the coupling constants found in platinum complexes with a cis arrangement of phosphine ligands are consistently 650-750 Hz smaller in magnitude. Solid state 31P-*Y5Pt coupling constants compare well with those found in solution IS]. lg5F’t NMR

Solid state 195Pt NMR is notoriously difficult for all but the most symmetric platinum com-

20

10

0

-10

PPm from 85% H3P04 Fig. 4. -?‘P MAS spectrum of the polymer 1. Satellites due to coupling to tpsPt yield an approximate value of 2440 Hz for the 3’P-‘y5Pt coupling constant. The broadness of the resonance is probably due to disorder amongst the phosphine ligands.

1

I

I

I

-4200

-4600

-5000

-5400

PPm Fig. 5. “‘sPt MAS spectrum of the dimeric precursor compound 2 recorded with spinning speeds of (a) 11 kHz and (b) 12.5 kHz. The chemical shift values are measured in ppm relative to [PtCl,]‘~ in D,O. The isotropic shift at -4420 ppm is labelled *. Note that the spinning sideband pattern extends beyond the limits of the experimental spectrum, and thus many sidebands are aliased. Each spinning sideband consists of a triplet due to coupling to two approximately equivalent slP (Jr_, = 2454 Hz).

pounds, because of the very large chemical shift anisotropies commonly associated with the 195Pt nucleus [14]. Very high spinning speeds are required to average out the shielding tensor as far as possible. Accordingly, all our ly5Pt spectra are recorded at spinning speeds in excess of 11 kHz. The spectra for the precursor 2 recorded with spinning speeds of 11 and 12.5 kHz are shown in Fig. 5. We have been unable to observe a resonance for the polymer 1. The large number of spinning sidebands resolved in both spectra have allowed the ‘95Pt shielding tensor to be determined. We used the graphical method of Herzfeld and Berger [15]; the large number of aliased sidebands precluded the use of the moments method proposed by Maricq and Waugh [16]. The results are:

16

M.J. Duer et al. /Solid State Nucl. Magn. Reson. 1 (1992) 13-16

u i,

port and Dhaka University (Bangladesh) for study leave (MSK), Dr. Malcolm Gerloch at the University of Cambridge and Prof. Todd. B. Marder at the University of Waterloo (Canada) for discussions. MJD thanks the Royal Society for NMR probe and for a 1983 University Research Fellowship.

=5225 k 60 ppm w22= 4760 + 60 ppm u s3 = 3280 f 60 ppm These compare well with the principal shielding values found for cis-Pt(PMe,),Cl, (ai, = 5058 ppm; uz2 = 4649 ppm; a,, = 4256 ppm) [141. The relative principal shielding values have been analysed using a parametric ligand field model, the Cellular Ligand Field Model [17]. Preliminary results suggest that (i> the Pt(II)-P a-bonds are very strong, and (ii) the acetylide and phosphine ligands both appear to act as r-acceptors. Full details of this analysis will form the subject of a forthcoming paper.

Conclusions

We have shown that the solid state NMR spectra of the rigid rod-like platinum a-acetylide complexes can be used to elucidate their electronic and geometric structure. The 13C-195Pt coupling constant yields valuable information about the hybridisation of the acetylide carbon r-bonded to the Pt(I1) ion in the dimeric complex 2. The 3’P-‘95Pt coupling constants verify the ligand geometry at platinum as tram in both the precursor and polymer complexes (1,2>. The lack of a 19’Pt spectrum and of an (Yacetylide resonance in the i3C spectrum for the polymer 1 are undoubtedly related phenomena. It is known that this polymer exhibits semi-conductor behaviour; it seems likely therefore that nuclear-electron spin interactions are causing very fast spin-lattice relaxation of both the 19’Pt and a-acetylide ‘sC spins. This would also explain the lack of scalar coupling between the 195Pt and a-acetylide 13C nuclei.

Acknowledgements

We are grateful to NSERC (Canada) for a post-doctoral fellowship (AKK), SERC for sup-

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5 B.F.G. Johnson, A.K. Kakkar, MS. Khan, J. Lewis, A.E. Dray, F. Wittmann and R.H. Friend, J. Mater. Chem., 1 (1991) 485. 6 B.F.G. Johnson, A.K. Kakkar, MS. Khan and J. Lewis, J. Organomet. Chem., 409 (1991) C12. 7 M.S. Khan, S.J. Davies, A.K. Kakkar, D. Schwartz, B. Lin, B.F.G. Johnson and J. Lewis, J. Organomet. Chem., (1991) in press. 8 J. Lewis. M.S. Khan, A.K. Kakkar, B.F.G. Johnson, T.B. Marder, H.B. Fyfe, F. Wittmann, R.H. Friend and A.E. Dray, J. Organomet. Chem.. (1991) submitted for pubhcation. 9 S. Guha, C.C. Frazier, P.L. Porter, K. Kang and S.E. Finberg, Optics Lett., 14 (1989) 952. 10 G.R. Williams, J. Phys. C, 21 (1988) 1971. 11 S.R. Phillpot, M.J. Rice, A.R. Bishop and D.K. Campbell, Phys. Rec.. B, 36 (1987) 1735. 12 W.J. Blau, H.J. Byrne, D.J. Cardin and A.P. Davey, .I. Mater. Chem., 1 (1991) 245. 13 P.S. Pregosin, Coord. Gem. Reel.. 44 (1982) 247 and references therein. 14 R.K. Harris and P. Reams, J. Chem. Sot. Dalton Trans.. (1986) 1015. 15 J. Herzfeld and A.E. Berger, J. Chem. Phys., 73 (1980) 1021. 16 M.M. Maricq and J.S. Waugh, J. Chem. Phys., 70 (1979) 3300. 17 M. Gerloch and R.G. Woolley, Prog. Inorg. Chem., 31 (1983) 371.