Reactivity and electrochemical properties of the d 8 /d 8 Pd 2 (dppm) 2 (Cl) 2 (Me) 2 towards MeI David Evrard a,b , Marc Drouin a ,Yves Mugnier b, *, Pierre D. Harvey a, * a De ´partement de Chimie, Universite ´ de Sherbrooke, Sherbrooke, PQ, Canada J1K 2R1 b Laboratoire de Synthe `se et d’E ´ lectrosynthe `se Organome ´talliques (LSEO-UMR 5632), Faculte ´ des Sciences Gabriel, 6 boulevard Gabriel, 21000 Dijon, France Received 12 July 2002; accepted 7 October 2002 This paper is dedicated to Professor Pierre Braunstein Abstract The title dimer reacts with MeI or KI to form [Pd 2 (dppm) 2 (m-I)(Me) 2 ]I. The latter has been characterized from X-ray crystallography, and exhibits a A-frame structure with a Pd /ÁÁÁ /Pd separation of 3.061 A ˚ . The 4-electron reduction of the d 8  /d 8 title starting material Pd 2 (dppm) 2 (Cl) 2 (Me) 2 and resulting [Pd 2 (dppm) 2 (m-I)(Me) 2 ]I product in the presence of dppm, produces the known Pd 2 (dppm) 3 dimer. The presence of intermediates such as the d 9  /d 9 ‘Pd 2 (dppm) 2 (Me) 2 ’ and d 10  /d 10 ‘Pd 2 (dppm)2(I) n n ’ species are suspected, and 31 P NMR indicates evidence for the relatively unstable mononuclear Pd(dppm)(Me) 2 complex. The reductive electrolysis of both d 8 /d 8 dimers in the presence of an excess of MeI leads to an increase in Pd(dppm)(Me) 2 concentration. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Electrolyses; Dinuclear; Electrochemical; X-ray crystallography 1. Introduction Palladium and its coordination complexes and clus- ters are well known for their importance in hetero- and homogeneous catalysis [1  /4]. Recently our groups have investigated a number of stoichiometric and catalytic electrochemical reactions such as CO 2 activation [5,6], C  /X bond cleavage (X /Br, I) [7],H 2 -evolution and formate decomposition [8], using various species of the type ‘Pd x (dppm) x ’ such as Pd(dppm)Cl 2 , Pd 2 (dppm) 2 Cl 2 , Pd 3 (dppm) 3 (CO) 2 , and Pd 4 (dppm) 4 (H) 2 2 (Scheme 1). With respect to CO 2 activation, while the electrochemical reductive coupling of H  and CO 2 to generate formate ion is well known, the C /C bond formation between CO 2 and an organic fragment is more rare [9]. In that context, the common approach is the Grignard-type electrochemical reaction (RMgX / CO 2  /2e  0/MgX   /RCO 2  ) [10]. Such an approach involves the anchoring of organic fragments and CO 2 simultaneously onto the Pd centre (or centres). Appro- priate candidate inorganic species are the d 8  /d 8 either as face-to-face [11  /15] or A-frame [16  /25] structures. Previous works have shown that electrochemical reduc- tion of Pd(dppm)Cl 2 and Pd 2 (dppm) 2 Cl 2 leads to the reactive zerovalent species Pd(dppm) l/ 1 2 /Pd 2 (dppm) 2 [6], which are responsible for CO 2 activation. The latter species has been observed earlier at low temperature by Stille et al. [26] by NMR spectroscopy, but no X-ray data have been available in this case. On the other hand, at least two other examples of zerovalent Pd 2 (diphos) 2 species have recently been characterized from X-ray crystallography (diphos /Cy 2 PCH 2 PCy 2 [15], iPr 2 PCH 2 PiPr 2 [27]). In addition, unsaturated zerova- lent Pd species are readily anticipated to undergo an oxidative addition towards halocarbons. In that respect, the Pd 2 (dppm) 2 species should react with MeI prior to the electrochemical event. Years ago, Balch et al. [28] * Corresponding authors. Tel./fax:  /33-3-80-39 6091 (Y.M.); Tel.:  /1-819-821 7092; fax:  /1-819-821 8017 (P.D.H.). E-mail addresses: yves.mugnier@u-bourgogne.fr (Y. Mugnier), pharvey@courrier.usherb.ca (P.D. Harvey). Inorganica Chimica Acta 350 (2003) 442  /448 www.elsevier.com/locate/ica 0020-1693/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0020-1693(02)01554-2