The efficient copolymerization of acrylates with ethylene using Ni catalysts remains a challenge. Herein, we report two neutral Ni­(II) catalysts (POP-Ni-py (1) and PONap-Ni-py (2)) that exhibit high thermal stability and significantly higher incorporation of polar monomer (for 1) or improved resistance to tert-butylacrylate (tBA)-induced chain transfer (for 2), in comparison to previously reported catalysts. Nickel alkyl complexes generated after tBA insertion, POP-Ni-CCO­(py) (3) and PONap-Ni-CCO­(py) (4), were isolated and, for the first time, characterized by crystallography. Weakened lutidine vs pyridine coordination in 2-lut facilitated the isolation of a N-donor-free adduct after acrylate insertion PONap-Ni-CCO (5) which represents a novel example of a four-membered chelate relevant to acrylate polymerization catalysis. Experimental kinetic studies of six cases of monomer insertion with aforementioned nickel complexes indicate that pyridine dissociation and monomer coordination are fast relative to monomer migratory insertion and that monomer enchainment after tBA insertion is the rate limiting step of copolymerization. Further evaluation of monomer insertion using density functional theory studies identified a cis–trans isomerization via Berry-pseudorotation involving one of the pendant ether groups as the rate-limiting step for propagation, in the absence of a polar group at the chain end. The energy profiles for ethylene and tBA enchainments are in qualitative agreement with experimental measurements.

High yield syntheses of ((i)Pr(3)P)(2)NiX (3a-c), (where X = Cl, Br, I) were established by comproportionation of ((i)Pr(3)P)(2)NiX(2) (1a-c) with ((i)Pr(3)P)(2)Ni(η(2)-C(2)H(4)) (2). Reaction of 1a with either NaH or LiHBEt(3) provided ((i)Pr(3)P)(2)NiHCl (4), along with 3a as a side-product. Reduction of ((i)Pr(3)P)(2)NiCl (3a-c) with Mg in presence of nitrogen saturated THF solutions provided the dinitrogen complex [((i)Pr(3)P)(2)Ni](2)(μ-η(1):η(1)-N(2)) (5). In aromatic solvents such as benzene and toluene a thermal equilibrium exists between 5 and the previously reported monophosphine solvent adducts ((i)Pr(3)P)Ni(η(6)-arene) (6a,b). Reaction of 5 with carbon dioxide provided ((i)Pr(3)P)(2)Ni(η(2)-CO(2)) (7). Thermolysis of 9 at 60 °C provided a mixture of products that included the reduction product ((i)Pr(3)P)(2)Ni(CO)(2) (8) along with (i)Pr(3)P=O, as identified by NMR spectroscopy. Complex 8 was also prepared in high yield from the reaction of 5 with CO. Reaction of 5 with CS(2) gave the dimeric carbon disulfide complex [((i)Pr(3)P)Ni(μ-η(1):η(2)-CS(2))](2) (9). Diphenylphosphine reacts with 5 to form the dinuclear Ni(I) complex [((i)Pr(3)P)Ni(μ(2)-PPh(2))](2) (10). Complex 5 reacts with PhSH to form ((i)Pr(3)P)(2)Ni(SPh)(H) (11), which slowly loses H(2) and (i)Pr(3)P to form the dimeric Ni(I) complex [((i)Pr(3)P)Ni(μ(2)-SPh)](2) (12) at room temperature. Complex 12 was also accessed by salt metathesis from the reaction of ((i)Pr(3)P)(2)NiCl (3a) with PhSLi, which demonstrates the utility of 3a as a Ni(I) precursor. With the exception of 6a,b, all compounds were structurally characterized by single-crystal X-ray crystallography.

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The activation and catalytic functionalization of CÀH bonds has gained importance as both a green and economical synthetic approach. [1] A modern goal is the use of less expensive first-row metals, such as nickel, in lieu of the expensive second-and third-row metals currently used for most catalytic C À H functionalization processes. Although there are examples of catalytic CÀH bond functionalization using nickel, which include unprecedented reactions such as CÀH bond stannylation, [2] these transformations are typically limited to activated substrates like fluorinated aromatics. [3] An alternative approach is to use cooperative bond activation [4] by dinuclear or polynuclear complexes in unusual oxidation states; for example, dinuclear Ni I complexes have been reported to mediate the rearrangement of activated CÀ H bonds. [5] Polynuclear clusters of the first-row transition metals may have an advantage over their heavier congeners in catalytic reactivity, because of the smaller HOMO-LUMO gap and weaker metal-metal bonds that render these complexes more reactive (HOMO/LUMO = highest occupied/ lowest unoccupied molecular orbital). [6] The reaction of [(iPr 3 P) 2 Ni] 2 (m-N 2 ) [7] and dihydrogen with the loss of N 2 according to Scheme 1 provides the pentanuclear cluster (iPr 3 P)Ni(m 3 -H) 2 [(iPr 3 P)Ni(m 2 -H)] 4 (1). Cooling a pentane solution of the product to À34 8C led to the precipitation of dark-brown rhombic crystals of 1 in 26 % yield; the low yield reflects the high solubility of the product. The infrared spectrum confirmed the presence of bridging hydride ligands and the absence of terminal hydrides. A broad band attributed to n(Ni À H) is found at 1235 cm Àl . The solidstate structure of 1 was determined by X-ray diffraction, and two views are given in Figure 1. [8] Complex 1 consists of a distorted square pyramid of Ni atoms, with the base atoms exhibiting marked differences in the Ni-Ni distances (2.3891(4)-2.6310(4) ); the bond lengths and angles associated with these distortions are shown in Figure 2. The electron densities associated with the six hydride ligands were located in a difference map, and the hydride positions were refined. Each basal Ni-Ni edge is spanned by a m 2 -hydride ligand. Hydrogen atoms H(4) and Scheme 1. Synthesis of pentanuclear complex 1. Figure 1. Solid-state molecular structure of [(iPr 3 P)Ni] 5 H 6 (1) as determined by X-ray crystallography, shown with 50 % probability ellipsoids. Hydrogen atoms not attached to Ni are omitted for clarity. Selected bond distances [] and angles [8]:

Nickel monophosphine arene adducts have been proposed as highly reactive intermediates capable of difficult C-O bond activation steps in Ni catalyzed cross-coupling reactions. The addition of N-methylmorpholine N-oxide to arene solutions of ([CyP)Ni]N allows for the synthesis of stable (η-arene)Ni(PCy) complexes. The isolation of these species demonstrates their viability as intermediates and provides an experimental means to test the hypothesized importance of the Ni(PCy) moiety in bond activation and catalysis.