Structural and Electronic Luminescence Studies of Trimetallic Gold-Copper N-Heterocyclic Carbene Complexes
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This thesis investigates the synthesis and characterization of various N-heterocyclic carbene trimetallic gold-copper complexes. Preparations include the synthesis of pyridine-functionalized benzimidazolylidene and imidazolylidene complexes with a core consisting of two Cu(I) ions and one Au(I) ion. In chapter two, the following pyridine-functionalized benzimidazolylidene complexes were synthesized from [Au(L1)2]PF6, (where L1 is the NHC, 1,3-bis(2-pyridinylmethyl)-1H-benzimidazolium hexafluorophosphate) complex 2.1, [AuCu2(L1)2(CH3CN)4](PF6)3, and complex 2.2, [AuCu2(L1)2(Me2CO)2](PF6)3. Complex 2.1 was found to have reversible mechanochromic properties changing from a blue luminescence, 467 nm, to a bright yellow luminescence, 532 nm, upon grinding. Complex 2.1 was also found to have irreversible thermochromic properties, when heated to 80 °C a change in luminescence occurred that was yellow to the eye, but gives a broad emission spectrum from 380 nm to about 530 nm. In complex 2.2, a major structural change is observed upon coordination of acetone to the copper centers with the introduction of short Au-Cu metallophilic interactions that were not observed in complex 2.1. In chapter three, a series of trimetallic gold-copper halide bridged pyridine-functionalized imidazolylidene complexes were synthesized from [AuCu2(L2)2(CH3CN)4](BF4)3 (where L2 is 1,3-dihydro-1,3-bis(2-pyridinylmethyl)-2H-imidazol-2-ylidene). Complexes 3.1 (containing Cl-), 3.2 (containing Br-), and 3.3 (containing I-) were found to have a rhombohedral core of one Au(I) ion, two Cu(I) ions, and one halide. There was a short Au(I)-Cu(I) metallophilic interaction in each case which were supported by the bridging halides and were similar in length, only increasing slightly from chloride (~2.77 Å) to bromide (~2.78 Å) to iodide (~2.81 Å). Complexes 3.1 and 3.2 had nearly identical solid state emissions at 538 and 537 nm, while complex 3.3 has a slightly red shifted λmax at 546 nm. In chapter four, a series of nitrile and alcohol complexes were synthesized to test how subtle changes in sterics and electronics affected the luminescence. All complexes were synthesized from [AuCu2(L2)2(CH3CN)4](PF6)3 (where L2 is 1,3-dihydro-1,3-bis(2-pyridinylmethyl)-2H-imidazol-2-ylidene). Complexes 4.1 (with valeronitrile) and 4.2 (with trimethylacetonitrile) followed the expected structural motif with no attractive interaction between the gold and coppers ,>4.5 Å, and a blue luminescence of 462 nm and 468 nm, respectively. Complex 4.3 (with chloroacetonitrile) had a green luminescence, 544 nm, but unexpectedly also had no attractive interaction between the gold and coppers, 4.599 Å. Complexes 4.4-4.6 (with n-butanol, isobutanol, and t-butanol, respectively) all luminesce bright green, but no crystal structures have been obtained to date. Instead, the solvato-species with an acetone in close proximity to each copper (4.7), and a two coordinate copper species with two gold-copper metallophilic interactions was obtained (4.8).