The stepwise synthesis of a palladium(II) bo&d altemating oligomer of CO and strained alkenes is described together with the factors that allow full control of each successive step. The sequence starts with acetylpalladium-(11) complexes PdX(COMe)(N-N) (2,4), (X = Cl(a), Br(b), or I(c); N-N = N,N,","-tetramethylethylenediamine (tmeda) or 2,2'-bipyridyl (bpy)). The bpy-coordinated acetyl complexes (4a-c) show an unexpected isomerization reaction of the groups around the metal center for which thermodynamic parameters have been determined. A mechanism for the isomerization is proposed. The ionic complexes [Pd(COMe)(MeCN)(N-N)]OTf (5, 6; OTf = trifluoromethanesulfonate), obtained in situ from reaction of 2a-c and 4a-c with AgOTf, react with a number of alkenes. Nonstrained alkenes do insert, but decomposition (probably involving B-elimination) is fast. Alkenes containing potential donor groups do not insert at all. Only in the case of strained alkenes like norbomene and its derivatives were the insertion products isolated and fully characterized by I3C NMR, IR, and X-ray analysis. Crystals of the dicyclopentadiene insertion product [Pd(C1$I12COMe)(bpy)]OTf (8c) are monoclinic, space group P21/n, a = 8.3086(4) A, b = 15.3894(6) A, c = 18.1479(9) A, B = 99.98(1)', V = 2285.36(19) A3, Z = 4, R = 0.042 (wR = 0.034). Here, the alkene insertion has occurred in cis,exo fashion on the norbomene-type alkene moiety, indicating this reaction to be both stereo-and chemoselective. The molecular structure shows that the acyl group is coordinated to the metal. Products resulting from a second CO insertion into the Pd-C bond of 8a, i.e. PdX(COC7HloCOMe)-(bpy) (X = C1 (12a) or I (12b)), were formed in the presence of a large excess of a sodium halide salt. Reaction of 12a,b with AgOTf and norbomene gave the alkyl complex [ P~( C~H I~C O C~H I~C O M~) (~~~) ] O T~ (Ua). Norbomadiene was inserted similarly. Attempted recrystallization of the insertion product of 5-norbomene-endu-2,3-dicarboxylic anhydride (13c) gave the alkyl complex 8a. This is the first example of a reversible alkene insertion reaction on an isolated palladium complex. Addition of CO and excess NaI (20 equiv) to 13a gave the neutral acyl complex P~I ( C O C~H~~C O C~H I~C O M~) (~~~) (14), the first isolated and fully characterized CO/alkene oligomer still connected to the palladium(I1) catalytic site. Crystals are triclinic, space group P i with a = 10.128(2) A, b = 11.655(2) A, c = 13.633(2) A, a = 109.89(1)', / 3 = 100.58(1)0, y = 92.92(2)', V = 1476.3(4) A3, Z = 2 and R = 0.044 (wR = 0.057). Norbomadiene also reacts with 2 equiv. of 5 or 6 to give the homoligated dinuclear complexes [(N-N)P~((C~H~)(COM~)~-~,~}P~(N-N)](OT~)Z with N-N = tmeda (9) or bpy (10). Crystals of 9 are orthorhombic, space group P212121, a = 11.176(3) A, b = 11.362(1) A, c = 30.394(3) A, V = 3859(1) A3, Z = 4, wR2 = 0.124, R = 0.049.In this complex a double alkene insertion reaction has taken place involving both alkene functionalities of a dialkene: 9 is symmetrically substituted with the ...

A n Aryllithium Complex with Intramolecular Coordination of an o-Diamine Substituent. X-ray and Solution NMR Structures of Dimeric [Li(CsH4(CH2N(Me)CH2CH2Ne2)=2}12

2)(O-t-Bu)2,3b, that is related to 3d but which contains only one o-amino substituent, can be prepared in a one-pot procedure by a sequence of transmetalation reactions starting from Ta(=CH-t-Bu)C13(THF)z (THF = tetrahydrofuran), Zn(CsH4CHzNMe2-2)2, and LiO-t-Bu. All alkoxide complexes are fluxional in solution at room temperature and undergo dissociation/ association processes involving the coordinative Ta-N bonds of the arylamine ligands.

The potentially C,N,N′-chelating anionic aryldiamine ligand [C 6 H 4 (CH 2 N(Me)CH 2 CH 2-NMe 2)-2]-(CNN) has been employed in the preparation of six-coordinate Ta(V) alkylidene complexes. The new dichloro alkylidene complex [TaCl 2 (dCH-t-Bu)(CNN)], 2, prepared from [TaCl 3 (dCH-t-Bu)(THF) 2 ] and [Li(CNN)] 2 , 1, has been obtained as a mixture of three diastereoisomers (2a-c) in 51% yield. The molecular structure of the major isomer 2a, determined by X-ray methods, shows this to be a tetragonal bipyramidal complex in which the metal coordination sphere is comprised of the C,N,N′ facially bonded CNN, two cispositioned chlorides in the equatorial plane, and a neopentylidene group and the NMe 2 nitrogen donor mutually trans-positioned at the apices. The dialkoxide complexes [Ta-(dCHR)(CNN)(O-t-Bu) 2 ] (R) t-Bu (3a), CMe 2 Ph (3b)) have been obtained by transmetallation of [TaCl(dCHR)(O-t-Bu) 2 (PMe 3)] with 1. The neopentylidene complex 3a was isolated as an analytically pure orange solid in 47% yield, whereas the neophylidene complex 3b was isolated as an orange oil of (90% purity. Complex 3b reacts in pentane with ethene at room temperature to afford the tantalacyclobutane complex [Ta{(CH 2) 3-1,3}(CNN)(O-t-Bu) 2 ], 4, which was isolated as a white solid in 81% yield. An X-ray diffraction study of 4 shows it to be a heptacoordinate pentagonal bipyramidal tantalum species in which two mutually trans alkoxide groups are in the apical positions; the meridional ligation is comprised of the C,N,N′-coordination of CNN and the σ-bonded carbon atoms of the metallacyle. Reaction of 3b in hexane at 69°C with ethene gives a mixture of the new tantalum alkene complex [Ta(CNN)(O-t-Bu) 2 (H 2 CdCH 2)], 5, and 4 in a 5:2 ratio. Complex 5 can be isolated from this mixture as a yellow solid by recrystallization from diethyl ether at-30°C. The X-ray molecular structure of 5 shows it to be a pentagonal bipyramidal complex in which the array of atoms directly bonded to tantalum is like that in 4. However, in 5, the meridional ligation is comprised of the C,N,N′-coordination of CNN and the two C atoms of the ethene molecule. Complexes 3b and 4 initiate ring-opening metathesis polymerization with strained cyclic olefins; dicyclopentadiene produces cross-linked polymers, whereas norbornene produces polycyclopentamers with approximately 90% trans-vinylene bonds. In contrast, none of the isomers of alkylidene complex 2 show any reactivity toward either ethene or norbornene.

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The insertion of both norbornene and carbon monoxide into the Pd-C bond of 2,2'-bipyridyl coordinated palladium(it) complexes can be controlled by the nature of the anions present in solution, and this allows the stepwise synthesis of a metal-bound COhorbornene co-oligomer of which an X-ray crystal structure is presented.

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Alkylidene-Centered Rearrangement of a Tantalum Alkylidene Alkoxide Species with the N,C,N-Bis-ortho-chelated 1,2,6-Trisubstituted Aryldiamine Ligand [C 6 H 3 (CH 2 NMe 2) 2-2,6]-to a Product with a C,N-Mono-ortho-chelated 1,2,4-Trisubstituted Aryldiamine Ligand. X-ray Molecular Structure of [TaCl(dCH-t-Bu){C 6 H 3 (CH 2 NMe 2) 2-2,4}(O-t-Bu)] †