Ligands, Lewis bases that coordinate to the metal center in a complex, can completely change the catalytic behavior of the metal center. In this Account, we summarize new reactions enabled by a single class of ligands, phosphine-sulfonates (ortho-phosphinobenzenesulfonates). Using their palladium complexes, we have developed four unusual reactions, and three of these have produced novel types of polymers. In one case, we have produced linear high-molecular weight polyethylene, a type of polymer that group 10 metal catalysts do not typically produce. Secondly, complexes using these ligands catalyzed the formation of linear poly(ethylene-co-polar vinyl monomers). Before the use of phosphine-sulfonate catalysts, researchers could only produce ethylene/polar monomer copolymers that have different branched structures rather than linear ones, depending on whether the polymers were produced by a radical polymerization or a group 10 metal catalyzed coordination polymerization. Thirdly, these phosphine-sulfonate catalysts produced nonalternating linear poly(ethylene-co-carbon monoxide). Radical polymerization gives ethylene-rich branched ethylene/CO copolymers copolymers. Prior to the use of phosphine-sulfonates, all of the metal catalyzed processes gave completely alternating ethylene/carbon monoxide copolymers. Finally, we produced poly(polar vinyl monomer-alt-carbon monoxide), a copolymerization of common polar monomers with carbon monoxide that had not been previously reported. Although researchers have often used symmetrical bidentate ligands such as diimines for the polymerization catalysis, phosphine-sulfonates are unsymmetrical, containing two nonequivalent donor units, a neutral phosphine, and an anionic sulfonate. We discuss the features that make this ligand unique. In order to understand all of the new reactions facilitated by this special ligand, we discuss both the steric effect of the bulky phosphines and electronic effects. We provide a unified interpretation of the unique reactivity by considering of the net charge and the enhanced back donation in the phosphine-sulfonate complexes.

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Aluminum and gallium amidinate complexes, {RC(NR‘)2}MMe2 (R, R‘ = alkyl; M = Al, Ga), react with the “cationic activators” [Ph3C][B(C6F5)4] and B(C6F5)3 to yield cationic Al and Ga alkyl species whose structures are strongly influenced by the steric properties of the amidinate ligand. The reaction of acetamidinate Al complexes {MeC(NR‘)2}AlMe2 (R‘ = i Pr, 1a; R‘ = Cy, 3a) with 0.5 equiv of [Ph3C][B(C6F5)4] or B(C6F5)3 yields {MeC(NR‘)2}2Al2Me3 + (R‘ = i Pr, 2a +; R‘ = Cy, 4a +) as the B(C6F5)4 - or MeB(C6F5)3 - salts. X-ray crystallographic analyses establish that 2a + and 4a + are double-amidinate-bridged dinuclear cations, in which the two metal centers are linked by μ-η1,η1 and μ-η1,η2 amidinate bridges. NMR studies show that 2a + undergoes two dynamic processes in solution:  (i) a μ-η1,η1/μ-η1,η2 amidinate exchange and (ii) Me exchange between the two metal centers. The reaction of {MeC(N i Pr)2}GaMe2 (1b) with 0.5 equiv of B(C6F5)3 yields {MeC(N i Pr)2}2Ga2Me3 + (2b +), whose structure and dynamic properties are similar to those of 2a +. The reaction of the bulkier t Bu-substituted amidinate complexes { t BuC(N i Pr)2}MMe2 (M = Al, 6a; M = Ga, 6b) with 0.5 equiv of [Ph3C][B(C6F5)4] yields { t BuC(N i Pr)2}MMe2·{ t BuC(N i Pr)2}MMe+ (M = Al, 7a +; M = Ga, 7b +) as the B(C6F5)4 - salts, the former of which is thermally unstable. An X-ray crystallographic analysis establishes that 7b + is a single-amidinate-bridged dinuclear cation, in which the two metal centers are linked by a μ-η1,η2 amidinate bridge. NMR data establish that the structures of 7a + and 7b + are similar and both species are rigid in solution. 6a and 6b also react with B(C6F5)3 to yield [7a][MeB(C6F5)3] and [7b][MeB(C6F5)3], respectively, which decompose by C6F5 - transfer to yield { t BuC(N i Pr)2}M(Me)(C6F5) (M = Al, 9a; M = Ga, 9b) and boron species. The “super-bulky” amidinate complexes { t BuC(N t Bu)2}MMe2 (M = Al, 12a; M = Ga, 12b) react with 1 equiv of [Ph3C][B(C6F5)4] to yield { t BuC(N t Bu)2}MMe+ (M = Al, 13a +; M = Ga, 13b +) as the B(C6F5)4 - salts. The salts [13a][B(C6F5)4] and [13b][B(C6F5)4] are thermally unstable and could not be isolated. However, the NMR data for 13a + and 13b + in C6D5Cl are consistent with base-free, three-coordinate structures or labile, four-coordinate solvated cations. These results provide a starting point for understanding the mechanism and reactivity trends in ethylene polymerization catalyzed by cationic Al amidinate species.

The synthesis and structures of mono- and bis(amidinate) aluminum complexes are described. The reaction of AlMe3 and 1 equiv of carbodiimide, R‘NCNR‘, affords {MeC(NR‘)2}AlMe2 (1a, R‘ = i Pr; 1b, R‘ = Cy = cyclohexyl). The reaction of R‘NCNR‘ with MeLi or t BuLi generates Li[RC(NR‘)2] (2a, R = Me, R‘ = i Pr; 3a, R = t Bu, R‘ = i Pr; 3b, R = t Bu, R‘ = Cy; 3c, R = t Bu, R‘ = SiMe3). 2a, 3a, and 3b may be isolated or reacted in situ, while attempted isolation of 3c gave [Li( t BuCN){μ-N(SiMe3)2}]2 (3d). The reaction of 1 equiv of AlCl3 with 2a or 3a − c affords {RC(NR‘)2}AlCl2 (4a, R = Me; 5a − c, R = t Bu), and the reaction of AlMe2Cl with 3a − c affords { t BuC(NR‘)2}AlMe2 (6a − c). Alkylation of 5a,b with 2 equiv of PhCH2MgCl or Me3CCH2Li yields { t BuC(NR‘)2}Al(CH2Ph)2 (7a,b) or { t BuC(NR‘)2}Al(CH2CMe3)2 (8a,b). The reaction of 0.5 equiv of AlCl3 with 2a or 3a,b yields {RC(NR‘)2}2AlCl (9a, R = Me; 10a,b, R = t Bu). Complexes 1b, 3d, 4a, 5a,b, 6b, 8b, 9a, and 10a,b have been characterized by X-ray crystallography. The crystallographic results establish that steric interactions between the R and R‘ groups influence the R‘−N−Al angle and, hence, the steric environment at aluminum.

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The synthesis, structures, and reactivity of cationic aluminum complexes containing the N,N'-diisopropylaminotroponiminate ligand ((i)Pr(2)-ATI(-)) are described. The reaction of ((i)Pr(2)-ATI)AlR(2) (1a-e,g,h; R = H (a), Me (b), Et (c), Pr (d), (i)Bu (e), Cy (g), CH(2)Ph (h)) with [Ph(3)C][B(C(6)F(5))(4)] yields ((i)()Pr(2)-ATI)AlR(+) species whose fate depends on the properties of the R ligand. 1a and 1b react with 0.5 equiv of [Ph(3)C][B(C(6)F(5))(4)] to produce dinuclear monocationic complexes [([(i)Pr(2)-ATI] AlR)(2)(mu-R)][(C(6)F(5))(4)] (2a,b). The cation of 2b contains two ((i)()Pr(2)-ATI)AlMe(+) units linked by an almost linear Al-Me-Al bridge; 2a is presumed to have an analogous structure. 2b does not react further with [Ph(3)C][B(C(6)F(5))(4)]. However, 1a reacts with 1 equiv of [Ph(3)C][B(C(6)F(5))(4)] to afford ((i Pr(2)-ATI)Al(C(6)F(5))(mu-H)(2)B(C(6)F(5))(2) (3) and other products, presumably via C(6)F(5)(-) transfer and ligand redistribution of a [((i)()Pr(2)-ATI)AlH][(C(6)F(5))(4)] intermediate. 1c-e react with 1 equiv of [Ph(3)C][B(C(6)F(5))(4)] to yield stable base-free [((i)Pr(2)-ATI)AlR][B(C(6)F(5))(4)] complexes (4c-e). 4c crystallizes from chlorobenzene as 4c(ClPh).0.5PhCl, which has been characterized by X-ray crystallography. In the solid state the PhCl ligand of 4c(ClPh) is coordinated by a dative PhCl-Al bond and an ATI/Ph pi-stacking interaction. 1g,h react with [Ph(3)C][B(C(6)F(5))(4)] to yield ((i)Pr(2)-ATI)Al(R)(C(6)F(5)) (5g,h) via C(6)F(5)(-) transfer of [((i)Pr(2)-ATI)AlR][(BC(6)F(5))(4)] intermediates. 1c,h react with B(C(6)F(5))(3) to yield ((i)Pr(2)-ATI)Al(R)(C(6)F(5)) (5c,h) via C(6)F(5)(-) transfer of [((i)Pr(2)-ATI)AlR][RB(C(6)F(5))(3)] intermediates. The reaction of 4c-e with MeCN or acetone yields [((i)Pr(2)-ATI)Al(R)(L)][B(C(6)F(5))(4)] adducts (L = MeCN (8c-e), acetone (9c-e)), which undergo associative intermolecular L exchange. 9c-e undergo slow beta-H transfer to afford the dinuclear dicationic alkoxide complex [(((i)Pr(2)-ATI)Al(mu-O(i)()Pr))(2)][B(C(6)F(5))(4)](2) (10) and the corresponding olefin. 4c-e catalyze the head-to-tail dimerization of tert-butyl acetylene by an insertion/sigma-bond metathesis mechanism involving [((i)Pr(2)-ATI)Al(C=C(t)Bu)][B(C(6)F(5))(4)] (13) and [((i)Pr(2)-ATI)Al(CH=C((t)()Bu)C=C(t)Bu)][B(C(6)F(5))(4)] (14) intermediates. 13 crystallizes as the dinuclear dicationic complex [([(i Pr(2)-ATI]Al(mu-C=C(t)Bu))(2)][B(C(6)F(5))(4)](2).5PhCl from chlorobenzene. 4e catalyzes the polymerization of propylene oxide and 2a catalyzes the polymerization of methyl methacrylate. 4c,e react with ethylene-d(4) by beta-H transfer to yield [((i)Pr(2)-ATI)AlCD(2)CD(2)H][B(C(6)F(5))(4)] initially. Polyethylene is also produced in these reactions by an unidentified active species.

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