Inorganica Chimica Acta, 76 (1983) L237-L238 L237 Binuclear Platinum Complexes with Polymethylene Bridges: a Neighbouring Atom Effect in Oxidative Addition P. K. MONAGHAN and R. J. PUDDEPHATF Department of Chemistry, University of W estern Ontario, London N6A 5B7, Ont., Canada Received November 24.1982 p-Methylene and p-polymethylene complexes of transition elements are of great current interest, for example, as models for intermediates in the Fischer- Tropsch synthesis [ zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 11. One route to such complexes involves reaction of a,o-dihalogenoalkanes with metal carbonyl anions or with other electron-rich metal centres, and such reactions may lead either to metal derivatives M(CH,),X (M = metal complex, X = halogen) or to M(CH,),M [2-41. We now report a system, based on oxidative addition to dimethyl- 1 JO-phenanthroline)platinum(II),(I), in which both types of complex can be isolated [2]. Kinetic studies now show that the metal atom in M(CH,),X activates the C-X bond to further oxidative addition, and this neighbouring atom effect has been determined quantitatively for the first time for the cases with n = 3-5. The new reactions are shown in eqns. (1) and (2) (m = 1JO-phenanthroline). (CHJ,l N\i /Me (1) + I( CH2)n l --A kz Pt N’ 1 ‘Me + (N;;<;:)nl (1) I I (Ha), n. 0 (III b), n = 1 (IIb), “S 1 (UC), n.2 (IId). “= 3 (Ile), n= 4 (IIf). n. 5 N-N A N hl \/ 1 1” (II) + (I) ------+ I- Pt- KHz),-,Pt,- l / \ Me Me Me Me (2) (IVa). n= 2 (iVb), n D 3 (IVC). n=4 (Ii’d). n I 5 When n = 3, 4 or 5 the reaction of (Z) with I(CH,)J in acetone generally gave a mixture of complexes (ZZ) and (Iv), which were easily separated since the binuclear derivatives (IV) are very sparingly +Author to whom correspondence should be addressed. soluble in all common organic solvents whereas (ZZ) are soluble in acetone, CH*CI, or CHC13. With a large excess of I(CH&.,I, only (ZZ) was formed and the binuclear complexes (IV) could also be prepared in very high yield from the pure complexes (ZZ) according to eqn. (2). Polymethylene bridged deriva- tives of platinum were not previously known [5]. When n = 2, the products were a mixture of complex (ZVa) and ethylene and (ZZa) rather than the expected (ZZc); complex (ZZa) was also formed by reaction of iodine with (Z). Finally, when n = 1, the products were a mixture of (ZZb) and (ZZZb) with slow subse- quent isomerisation of the cis adduct (ZZZb) to the trans adduct (ZZb). Very little or no cis oxidative addition was observed in other cases. The composi- tions of derivatives (II), (ZZZ) and (IV) were deter- mined by full elemental analysis and by mass spectrometry, and the stereochemistries of the soluble complexes (ZZ) and (ZZZ) were determined by the characteristic ‘H NMR spectra [6,7]. The stereochemistry of (IV), n = 5, was determined by ‘H and 13C{lH} NMR spectroscopy* but, when n = 2-4, the complexes were insufficiently soluble for NMR characterization. Kinetic studies were carried out by monitoring the decay of the MLCT band in the W-visible spectra due to complex (Z) at 473 nm in acetone solution. When n = 3-5, the reactions (1) and (2) were first order in both reagents, using conditions in which a large excess of I(CH2)J or complex (ZZ) respectively was used. When n = 1, reaction (1) did not follow second order kinetics, but there was an induction period followed by a relatively fast reaction as expected for a free radical chain reaction, and reaction (2) did not occur. When n = 2, only an overall rate could be measured using excess ICH,CH,I; this was first order in each reagent in the early stages of reaction but the kinetics then became more complex. Resulting rate constants are given in Table I, together with comparative data for related alkyl iodides. The reactivity series (Me1 %-Et1 > PrI 9 IPrI) and observation of good second order kinetics strongly suggests the SN2 mechanism of oxidative addition for the n-alkyl iodides [8]. The expected low reactiv- ity of CH,Iz to nucleophilic attack [9] presumably accounts for it being forced to adopt the free radical mechanism of oxidative addition**, but the low *‘H NMR (CDzClz): [s, 70, MePt]; 1.42 *J(PtH) 0.96 PtH) 70, C?H2]; -0.07 [q, CPH2]; 0.40 ‘H} NMR: -5.45 [‘J(PtC) 700 MePt]. C?]; 29.18 [*J&W) 80, Cpi; 30.5i ICT. -**it is significant that this was the only reaction to give appreciable cis oxidative addition. The mechanistic basis for this effect is under investigation. 0020-l 693/83/0000-0000/$03 .OO 0 Elsevier Sequoia/Printed in Switzerland