A Novel Redox-Active Conjugated Palladium Homobimetallic Complex

Abstract

The π‐conjugated molecule N,N′‐bis(4′‐dimethylaminophenyl)‐1,4‐benzoquinone diimine (L2) was incorporated into the palladium(II) complex 1 bearing the N,N′‐bis(2‐phenylethyl)‐2,6‐pyridinedicarboxamide ligand, to afford the redox‐active conjugated palladium(II) homobimetallic complex 2. An X‐ray crystal structure determination of 2 reveals that two palladium complex units are bridged by the quinone diimine moiety to form the C2‐symmetrical 2:1 complex with an anti configuration, and that the bridging π‐conjugated spacer moieties are aligned along a straight line almost parallel to the a axis, to form the columns of the π‐conjugated molecules in the molecular packing. Variable temperature 1H NMR studies of the conjugated complex 2 indicate that the syn configuration is enthalpically more favorable than the anti configuration in CD2Cl2 by 1.0 kcal/mol, but entropically less favorable by 4.5 cal/mol from the van’t Hoff plot. The redox function of the quinone diimine moiety is modulated by complexation with the palladium complex 1. The conjugated complex 2 shows three separate redox waves assignable to the successive one‐electron reduction of the quinone diimine moiety and one‐electron oxidation process of the two terminal dimethylamino groups. Chemical reduction of 2 in THF with CoCp2 resulted in the appearance of ESR signals with weak 105Pd coupling centered around g = 2.0041. The added electrons are considered to be delocalized over the PdII‐quinone diimine d‐π* system in the complex.

Conjugated Complexes Composed of Quinonediimine and Palladium: Controlled Formation of a Conjugated Trimetallic Macrocycle

“…Treatment of L with [PdCl 2 (MeCN) 2 ] in acetonitrile led to the formation of the conjugated polymeric complex 2 , in which palladium centers are incorporated in the main chain (Scheme ). The electronic spectrum of 2 in DMSO exhibited a strong, broad absorption around 600–1000 nm, probably assignable to a low‐energy charge‐transfer transition with significant contribution from palladium; this indicates the coordination of the quinonediimine nitrogen atoms to palladium centers 5. The broad absorption band supports the formation of the conjugated polymeric complex 2 6.…”

“…Transition metal directed assembly is regarded as a useful approach to an architecturally controlled formation of conjugated polymeric or macrocyclic complexes with π‐conjugated ligands 4. We have already demonstrated that the redox‐active π‐conjugated ligand N , N ′‐bis(4‐dimethylaminophenyl)‐1,4‐benzoquinonediimine (L) serves as a bridging ligand to afford the conjugated homobimetallic palladium( II ) complex [L′PdLPdL′] ( 1 ) with the palladium( II ) complex bearing the N‐heterocyclic tridentate podand ligand N , N ′‐bis(2‐phenylethyl)‐2,6‐pyridinedicarboxamide (L′) 5. Complex 1 is present as a mixture of syn and anti isomers in dichloromethane.…”

Quinone Oxygen-Coordinated Palladium(II) Complexes with Anthraquinone Ligands BearingN-Heterocyclic Coordination Sites

“…Ϫ 1 H NMR (400 MHz, CD 3 CN): δ ϭ 3.17 (t, J ϭ 6.9 Hz, 4 H), 3.76 (q, J ϭ 6.9 Hz, 4 H), 7.15 (dd, J ϭ 8. 5,1.0 Hz,2 H),7.21 (ddd,J ϭ 7.7,4.9,1.0 Hz,2 H),7.32 (ddd,J ϭ 7.7,1.0,0.8 Hz,2 H),7.45 (dd,J ϭ 7.5,1.0 Hz,2 H),7.55 (dd,J ϭ 8.5,7.5 Hz,2 H),7.69 (td,J ϭ 7.7,1.9 Hz,2 H),8.54 (ddd,J ϭ 4.9,1.9,0.8 Hz,2 H),9.70 (br,2 H H 5.51, N 12.25; found C 73.29, H 5.33, N 12.15.…”

“…[3] However, the p-quinone complexes, in which a quinone oxygen coordinates to a high valent transition metal, have not been investigated extensively, [4] despite the elucidation of their coordination and redox properties in catalysis. In our previous papers, [5] the π-conjugated molecule, N,NЈ-bis(4Ј-dimethylaminophenyl)-1,4-benzoquinone diimine, was introduced to the palladium(II) complex bearing the N-heterocyclic tridentate podand ligand, affording a redoxactive π-conjugated palladium(II) homobimetallic complex, in which the redox properties of the quinone diimine moiety are modulated by complexation. These results prompted us to investigate the redox properties of the transition metal complexes with p-quinones and to characterize the complexes structurally.…”

A 3D Interlocked Structure from a 2D Template: Structural Requirements for the Assembly of a Square‐Planar Metal‐Coordinated [2]Rotaxane

“…[8] The square-planar [2]rotaxane ligand design consists of a tridentate benzylic amide macrocycle and a monodentate thread (Figure 1 d). The macrocycle incorporates a 2,6dicarboxyamidopyridine unit to exploit the palladium chemistry recently developed [9] by Hirao and co-workers. The thread contains a pyridine donor group substituted with appropriately bulky stoppers in either the 2,6-or 3,5-positions.…”