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Binuclear Palladium Complexes Supported by Bridged Pincer Ligands
journal contribution
posted on 2016-02-22, 10:52 authored by David
E. Herbert, Oleg V. OzerovA series of binucleating ligands each containing two
tridentate
pincer sites based on a central, monoanionic aryl (C) donor flanked
by neutral phosphinito (P) and imino (N) donors, [(PCN)-(CH2)n-(PCN)] (1), or a central,
monoanionic amido (N) flanked by neutral phosphine (P) and imino donors
(N), [(PNN)-(CH2)n-(PNN)] (4), are presented. The metalating sites were linked through
the condensation of two equivalents of m-hydroxybenzaldehyde
or the purposively built asymmetric diarylamine [(H)N(2-C(O)H-4-Me-C6H3)(2-P(iPr)2-4-Me-C6H3)] (3) with primary α,ω-diamines
(1,2-diaminoethane, n = 2; 1,4-diaminobutane, n = 4), which simultaneously constructed the imino arms
and bridged the pincer cores. Ligands 1 and 4 were then used in the synthesis of neutral, square-planar palladium(II)
complexes 5 and 6, [(PCN-Cn)Pd2X2, (PNN-Cn)Pd2X2; n = 2, 4; X
= Cl, OAc, OTf]. The difference in the trans influence
of the central donor was demonstrated through X-ray crystal structures
of 5b-Cl, (PCN-C4)Pd2Cl2, and 6b-Cl, (PNN-C4)Pd2Cl2. The (PNN-Cn)Pd2X2 complexes (6) proved to be redox-active, presumably
via oxidation of the ligand, and cyclic voltammetry illustrated the
extent to which electronic communication between the two pincer sites
is mediated by the length of the bridge between them. (PCN-Cn)Pd2OTf2 (5-OTf) and (PNN-Cn)Pd2OTf2 (6-OTf) complexes reacted with hydride donors such
as Et3SiH or β-hydride-containing alkoxides such
as NaOiPr to generate bridging-hydride monocations
[(PCN)Pd-H-Pd(PCN)-Cn][OTf] (5-H, n = 2, 4) and [(PNN)Pd-H-Pd(PNN)-C2][OTf] (6a-H), where the supported Pd–Pd separation
was also found to be affected by the bridge length. In the case of
the (CH2)4-bridged (PNN-C4)PdOTf
(6b-OTf), a bridging-hydride monocation was not observed.
Instead, the formation of [(PN(H)N)Pd-Pd(PNN)-C4][OTf]
(6b-H), protonated at the amido N of the ligand with
a direct Pd(I)–Pd(I) bond, was recorded.
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Keywords
amido Nmonoanionic arylcyclic voltammetrytrans influencebridge lengthBinuclear Palladium ComplexesLigands 1PCNPNOTfimino donorsmetalating sitesmonoanionic amidobinucleating ligandspincer sitesCHmonocationimino armsPdhydride donorstridentate pincer sitescomplexpincer coresBridged Pincer LigandsA seriesPNNEt 3SiH
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