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This report describes the synthesis, characterization, and reactivities of a new series of pincer-type nickel complexes based on the diphosphinito (POCOP) ligands 1,3-(i-Pr2PO)2C6H4, 1, and (i-Pr2POCH2)2CH2, 2. Reacting these ligands with (THF)1.5NiCl2, (THF)2NiBr2, or (CH3CN) n NiX2 (X = Br, n = 2; I, n = 3) gives pincer-type complexes by metalation of the central carbon atom. The yields of these (POCOP)NiX complexes vary with the type of ligand and Ni precursor used, as well as the reaction conditions. In general, the aromatic ligand 1 was metalated more readily to give excellent yields of the pincer complexes {2,6-(i-Pr2PO)2C6H3}NiX (X = Cl, 1a, 85% yield; X = Br, 1b, 95% yield; X = I, 1c, 85% yield), especially when the reaction mixture was heated to 60 °C for 1 h in the presence of 1 equiv of 4-dimethylaminopyridine (DMAP). The analogous reactions of ligand 2 were more sluggish and required refluxing in toluene to give {(i-Pr2POCH2)2CH}NiX (X = Cl, 2a, 33% yield; X = Br, 2b, 93% yield; X = I, 2c, 70% yield). Displacement of Br from 1b and 2b by Ag(O3SCF3), acetonitrile, or acrylonitrile gave, respectively, the neutral Ni−O3SCF3 derivatives 1d and 2d and the cationic adducts of CH3CN (1e and 2e) and CH2CHCN (1f and 2f). Reacting 1b with MeMgCl or EtMgCl gave the corresponding Ni−alkyl derivatives 1g and 1h, respectively, whereas alkylation of complexes 2a−c was unsuccessful. The POCsp3OP-based complexes 2a and 2b could be oxidized to paramagnetic, 17-electron species {(i-Pr2POCH2)2CH}NiIIIX2 (X = Cl, 2i; Br, 2j). Solid-state structures are reported for Ni−halide derivatives, the neutral complexes 2d, 1g, and 1h, the cationic adduct 1e, and the Ni(III) derivative 2i. The cationic acrylonitrile derivative 1f promotes the Michael addition of morpholine, cyclohexyl amine, or aniline to acrylonitrile, methacrylonitrile, or crotonitrile, whereas the paramagnetic NiIII complex 2j promotes the addition of CCl4 to methyl acrylate, methyl methacrylate, styrene, 4-methylstyrene, acrolein, and acrylonitrile (Kharasch reaction).

The coordination chemistry of a potentially pincer-type dicationic meta-phenylene-bis(imidazoliophosphine) ligand 3 to neutral and cationic carbonylrhodium(I) centers has been investigated. Similarly to what was observed previously for its ortho-phenylene counterpart, 3 was found to bind to the RhCl(CO) fragment in a trans-chelating manner that makes possible a weak Rh-C(H) interaction, inferred from the nonbonding but relatively short Rh-C and Rh-H contacts observed in the solid state structure of the dicationic adduct (3)RhCl(CO) (5). Formation of the target PCP-type pincer complex could not be triggered despite multiple attempts to deprotonate the central C-H moiety in the initial dicationic adduct 5, or in the tricationic species [(3)Rh(CO)](+) (8) generated by abstraction of the chloride ion from 5. Complex 8 was identified on the basis of NMR and IR analyses as a Rh(I)-stabilized P(CH)P-intermediate en route to the anticipated classical PCP-type pincer complex. Analysis of the electronic structure of this intermediate computed at the density functional level of theory (DFT level) revealed a bonding overlap between a Rh d-orbital and π-orbitals of the m-phenylene ring. NBO analyses and calculated Wiberg indices confirm that this interaction comprises an η(1)-C-Rh bonding mode, with only secondary contributions from the geminal C and H atoms. Although the target PCP-type pincer complex could not be generated, treatment of the tricationic intermediate with methanol induced a P-CN(2) bond cleavage at both imidazoliophosphine moieties, resulting in the formation of a dicationic "open pincer" species, that is, a nonchelated bis((MeO)PPh(2))-stabilized aryl-Rhodium complex that is the κC-only analogue of the putative κP,κC,κP-PCP complex sought initially. Theoretical studies at the DFT level of experimental or putative species relevant to the final C-H activation process ruled out the oxidative addition pathway. Two alternative pathways are proposed to explain the formation of the "open pincer" complex, one based on an organometallic α-elimination step, the other based on an organic aromatization-driven β-elimination process.

Unsymmetrical POC(H)N-type pincer ligands react with NiBr2(NCMe) x to give the complexes (POCN)NiIIBr (POCN= κ P ,κ C ,κ N -{2-(i-Pr2PO),6-(NR2CH2)-C6H3}; NR2= 3-morpholino (3a), NMe2 (3b), and NEt2 (3c)). The presence of an added base such as NEt3 in these metalation reactions maximizes the yields of the target pincer complexes by suppressing the formation of side-products arising from protonation of the ligand by the in situ-generated HBr. The cyclic voltammograms of 3 exhibit a quasi-reversible, one-electron oxidation at ca. +1.0 V, in addition to an irreversible oxidation at higher potentials likely due to ligand oxidation. Reaction of the 16-electron, yellow complexes 3 with Br2, N-bromosuccinimide, or CBr4 gives black crystalline compounds identified as the 17-electron complexes (POCN)NiIIIBr2 (5). Complexes 3 and 5 adopt square-planar and distorted square-pyramidal geometries, respectively. The Ni−Br bond lengths in 5 are significantly longer for the Br atom occupying the axial position (2.43−2.46 vs 2.37 Å), consistent with the repulsive interactions expected between the bromide lone pair and the singly occupied d z 2 /dpz hybrid MO. In agreement with this picture, the g-values obtained from the EPR spectrum of 5a are g xx ≈ g yy ≈ 2.2, g zz ≈ 2.00, and strong Br hyperfine coupling is observed on the g zz component. Our preliminary studies indicate that the thermal stabilities of 5, both in the solid state (as probed by differential scanning calorimetry) and in solution (as probed by UV−vis spectroscopy), vary in the order 5a ≈ 5b > 5c; this order of stability is consistent with the relative steric demands of the N-moiety. Complexes 5a (or 3a) and 5b (or 3b) promote the Kharasch addition of CX4 (X = Cl, Br) to styrene at 80 °C, giving PhC(X)CH2CX3, the product of an anti-Markovnikov addition, in up to 50 catalytic turnovers.

The role of methylaluminoxane (MAO) in the Ni-catalyzed dehydrogenative homologation of PhSiH3 has been investigated with a view to designing new cocatalysts possessing well-defined chemical compositions and structures. These studies show that species such as the bifunctional reagent (Me2PCH2AlMe2)2, 3, should act as co-catalyst for the Si-Si bond formation reactions. Thus, it was found that the combination of (1-Me-indenyl)Ni(PPh3)Me, 2a, and 3 (Ni/Al ratio of 1:1) converts PhSiH3 to cyclic oligomers (PhSiH)n with a turnover frequency (TOF) of >500 h(-1), 50 times faster than with 2a alone. Detailed NMR studies have indicated that this acceleration is due to the formation of the intermediate (1-Me-indenyl)Ni(Me)(Me2PCH2AlMe2), 4. Coordination of the PMe2 moiety in this complex to the Ni center allows the tethered AlMe2 moiety to interact with the Ni-Me moiety in such a way that promotes fairly slow Al-Me/Ni-CD3 exchange (t(1/2) ca. 12 h) but accelerates the Si-H bond activation and Si-Si bond formation reactions. The catalysis promoted by 2a/3 proceeds even faster in the presence of NEt3 or THF (TOF > 1600 h(-1)), because these Lewis bases favor the monomeric form of 3, which in turn favors the formation of 4. On the other hand, the much more nucleophilic base quinuclidine suppresses the catalysis (TOF < 300 h(-1)) by hindering the Ni.R.Al interactions. These observations point to an emerging strategy for using bifunctional reagents such as 3 to place geometrically constrained Lewis acid moieties adjacent to metal centers, thereby activating certain metal-ligand bonds.