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The ligand precursor [LH2]2+·2Br- (1) for the bis(NHC) ligand with N-picolyl moieties is synthesized in 83% yield. Complexation of 1 with palladium and nickel acetates produced [PdL]2+·2Br- (2) and [NiL]2+·2Br- (3), respectively, in quantitative yields. Metathesis reactions of 2 and 3 with AgPF6 give the corresponding bromide-free complexes [PdL]2+·2PF6 - (4) and [NiL]2+·2PF6 - (5). Complexes 2−4 were characterized by X-ray structural determinations, which reveal a highly twisted helical coordination of L around the metal ions. In solution, however, the tetradentate chelate ring undergoes a rapid fluxional process of ring twisting. A theoretical study confirms that the tetradentate coordination of L in 2 and 3 is energetically more favorable than the bidentate chelation mode with dangling picolyl groups. A preliminary application of [NiL]2+·2Br- in Suzuki coupling of aryl halides with phenylboronic acid shows effective activity.

The silver complex of PC NHC P was identified as [Ag 3 (µ-Cl)(PC NHC P) 2 ]Cl 2 (1). Anion exchange of 1 with AgNO 3 affords a white compound, which crystallizes into two forms, [Ag 3 (µ-Cl)-(PC NHC P)] 2 (NO 3 ) 2 (2a) and [Ag 3 (Cl)(NO 3 )(0.5DMF)(PC NHC P) 2 ]NO 3 (2b), from different solvent combinations. Their structural determination reveals interesting structural features, including a short Ag‚‚‚Ag interaction in both of them. Unlike the common preparation of ruthenium(II) NHC complexes in the literature, a novel silver carbene transfer reaction between 1 and RuCl 2 (PPh) 3 produces fac-[Ru 2 (µ-Cl) 3 (PC NHC P) 2 ]Cl (3) in good yield. The nonredox nature of the carbene transfer reaction is different from that of an earlier report and shows that the preparation of divalent ruthenium NHC complexes via silver carbene transfer is a feasible synthetic route. Significantly, complex 3 can also be prepared directly from PC NHC P‚HCl and RuCl 2 (PPh 3 ) 3 in the absence of base. Reactions of 3 with CO, phenylacetylene, and pyridine afford mer complexes mer,cis-RuCl 2 (CO)(PC NHC P) (4), mer-,trans-RuCl 2 (dCdCHPh)(PC NHC P) (6), and mer,trans-[RuCl(py) 2 (PC NHC P)]Cl (7), respectively. The fac complex, fac,cis-[RuCl(bipy)(PC NHC P)]Cl (8), was produced in reaction of either 3 or 7 with 2,2′-bipyridine. Treatment of 4 with excess NaBH 4 produced the hydride complex mer-RuHCl(CO)(PC NHC P) (5). Complex 7 is slowly oxidized to mer-[Ru III Cl 3 (PC NHC P)] ( 9) in the air. The structures of 2a, 2b, 3, 4, 7, 8, and 9 have been confirmed by X-ray crystallography. The reactivity study shows that PC NHC P is capable of accommodating extra steric requirement by readily switching from facial to meridional coordination mode, and the preliminary catalytic study indicates that 3 is effective in transfer hydrogenation of ketones.

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