Dinuclear Triazole-Derived Janus-Type N‑Heterocyclic Carbene Complexes of Palladium: Syntheses, Isomerizations, and Catalytic Studies toward Direct C5-Arylation of Imidazoles

The
dipalladium triazolidine-diylidene complex all-trans-[PdBr2(CH3CN)]2(μ-ditz) (1) (ditz = 1,2,4-trimethyltriazolidine-3,5-diylidene) was
synthesized via in situ deprotonation of the precursor
salt with a basic metal precursor. Ligand replacements of all-trans-1 with
monodentate or chelating phosphines afforded the dicarbene-bridged
complexes all-cis-[PdBr2(PPh3)]2(μ-ditz) (2) and [PdBr­(DPPP)]2(μ-ditz)­Br2 (3), respectively.
Bromido substitution of all-cis-2 gave
tetra-acetato complex all-cis-[Pd­(CH3COO)2(PPh3)]2(μ-ditz) (4) with retention of the configuration as the predominant product.
In addition, monopalladium triazolin-5-ylidene complexes trans-[PdBr2(CH3CN)­(tazy)] (6, tazy
=1,4-dimethyltriazolin-5-ylidene), cis-[PdBr2(PPh3)­(tazy)] (7), [PdBr­(DPPP)­(tazy)]­Br
(8), and cis-[Pd­(CH3COO)2(PPh3)­(tazy)] (9) were also synthesized
as the respective mononuclear equivalents for comparison. A comparative
catalytic study revealed the general superiority of dinuclear complexes 1–4 over their respective mononuclear
counterparts 6–9 in the direct C5-arylation
reaction of 1-methylimidazoles. Overall, mixed dicarbene/diphosphine
complex 3 showed the best catalytic performance.

通过在位去质子化前驱体盐，以碱性金属前驱体为媒介，合成了全反式二全桥联三氮唑二亚胺基二[二溴(三氟甲烷腈)](μ-ditz) (1) (ditz = 1,2,4-三甲基三氮唑-3,5-二亚胺基)。将全反式-1中的配体替换为单齿或螯合膦，得到了双卡宾桥连的复合物全顺式-[二溴(三苯基膦)]2(μ-ditz) (2)和[PdBr(DPPP)]2(μ-ditz)Br2 (3)。全顺式-2的溴代反应产生了四乙酸酯复合物全顺式-[二溴(乙酸甲酯)(三苯基膦)]2(μ-ditz) (4)，并保留了构型，作为主要产物。此外，还合成了相应的单核等价物，包括顺式-[二溴(三氟甲烷腈)(1,4-二甲基三氮唑-5-基)] (6, tazy = 1,4-二甲基三氮唑-5-基)，顺式-[二溴(三苯基膦)(1,4-二甲基三氮唑-5-基)] (7)，[二溴(三苯基膦)(1,4-二甲基三氮唑-5-基)]Br (8)和顺式-[二溴(乙酸甲酯)(三苯基膦)(1,4-二甲基三氮唑-5-基)] (9)。通过比较催化研究，揭示了双核复合物1-4相对于各自的单核对应物6-9在1-甲基咪唑的直接C5-芳基化反应中具有普遍的优越性。总体而言，混合双卡宾/二膦复合物3展现了最佳的催化性能。