UC Berkeley

Chapter 1. Multimetallic moieties are widely invoked as the active sites of enzymes and heterogeneous materials responsible for the facilitation of chemical transformations that, to date, are very challenging to achieve in research laboratories or industrial settings under mild conditions. Attention in the field has largely been focused on the investigation of these difficult-to-study systems by developing relatively simpler molecular analogues. The close coordination of multiple metal centers is thought to allow for the hypothesized metal-metal cooperativity by enabling multielectron processes through electronic interactions and coordinating/activating substrates. Systems have been designed to exploit cooperativity between transition-metal pairs, which can lead to unusual reactivity or selectivity. This chapter briefly summarizes historical and contemporary achievements in this area of research with special attention focused on the utilization of the 1,8-naphthyridine motif.Chapter 2. A dinucleating 1,8-naphthyridine ligand featuring fluorene-9,9-diyl-linked phosphinoside arms (PNNPFlu) was synthesized and used to obtain the cationic dicopper complexes: 2.2, [(PNNPFlu)Cu₂(μ‐Ph)][NTf₂]; 2.6, [(PNNPFlu)Cu₂(μ‐CCPh)][NTf₂]; 2.3, [(PNNPFlu)Cu₂(μ‐OtBu)][NTf₂]. Complex 2.3 reacted with diboranes to afford dicopper μ‐boryl species (2.4, with μ‐Bcat; cat = catecholate and 2.5, with μ‐Bpin; pin = pinacolate) that are more reactive in C(sp)–H bond activations and toward activations of CO₂ and CS₂, compared to dicopper μ‐boryl complexes supported by a 1,8-naphthyridine-based ligand with di(pyridyl) side arms. The solid-state structures and DFT analysis indicate that the higher reactivity of 2.4 and 2.5 relates to changes in the coordination sphere of copper, rather than to perturbations on the Cu–B bonding interactions. Addition of xylyl isocyanide (CNXyl) to 2.4 gave 2.7, [(PNNPFlu)Cu₂(μ‐Bcat)(CNXyl)][NTf₂], demonstrating that the lower coordination number at copper is chemically significant. Reactions of 2.4 and 2.5 with CO₂ yielded the corresponding dicopper borate complexes: 2.8, [(PNNPFlu)Cu₂(μ‐OBcat)][NTf₂]; 2.9, [(PNNPFlu)Cu₂(μ‐OBpin)][NTf₂]. Complex 2.4 demonstrated catalytic reduction in the presence of excess diborane. Related reactions of 2.4 and 2.5 with CS₂ provided the insertion products: 2.10, [(PNNPFlu)Cu₂]₂[μ‐S₂C(Bcat)₂][NTf₂]₂; 2.11, [(PNNPFlu)Cu₂(μ,κ²‐S₂CBpin)][NTf₂], respectively. These products feature Cu–S–C–B linkages analogous to those of proposed CO₂ insertion intermediates.Chapter 3. The selective synthesis and isolation of homo- and heterobimetallic Fe/Mn complexes was facilitated by the synthesis of a new symmetrical 1,8-naphthyridine ligand featuring bioinspired triazole side arms (MTN). This ligand was used to obtain the cationic diiron complex: 3.4, [(MTN)₂Fe₂(μ-Cl)(THF)₂][NTf₂]₃, and monometallic iron complex: 3.5, MTN·FeCl₂. The vacant coordination position in 3.5 was used as a starting point for the coordination of an additional metal center. The addition of one equivalent of Mn[OTf]₂(MeCN)₃ and Fe[OTf]₂ to 3.5 filled the vacant coordination position to yield: 3.6, [(MTN)₃Fe(μ-Cl)₂Mn][OTf₂]₂, 3.7, [(MTN)₃Fe₂(μ-Cl)₂][OTf₂]₂. The identity and characterization of 3.4, 3.5, and 3.7 are supported by mass spectrometry, X-ray diffraction analysis, and EPR spectroscopy. Furthermore, the identity of the metal center (3.6), and the ligand field environment (3.4), can both lead to varied magnetic behavior compared to the prototypical example of 3.7, likely originating from the particular exchange pathway of these materials, as investigated by variable-temperature magnetic susceptibility measurements.Chapter 4. Reaction of a dicopper(I) acetonitrile complex resulted in the activation of elemental sulfur to yield a formally 2Cu¹:2Cu² tetracopper mixed-valent μ-disulfide complex (4.1). Furthermore, stoichiometric reducing (cobaltocene) or oxidizing (Ag[NTf₂]) agents allow for the reversible interconversion between complex 4.1, a formally 3Cu¹:Cu² species (4.2), and a 4Cu¹ species (4.3). Given the isostructural nature of these complexes, the electronic communication between the metal centers of 4.1, 4.2, and 4.3 were spectroscopically investigated with UV-Vis and EPR techniques, revealing the relative delocalization of the electron holes on all four copper centers of the complexes in 4.1 and 4.2. Variable magnetometry of 4.1, 4.2, and 4.3 also reveals antiferromagnetic, ferromagnetic, and temperature-independent paramagnetic interactions, respectively. This study provides insight into related biological mixed-valent multicopper systems, shedding light on the unique electronic exchange interactions between the metal atoms.Chapter 5. A novel scaffold, 2,7-bis(2-fluorophenyl)-1,8-naphthyridine (5.1), was synthesized via Suzuki cross-coupling methods, allowing for facile access to new potential binucleating ligands. A variety of donor atoms can be furnished utilizing a straightforward nucleophilic aromatic substitution reaction. This versatility was demonstrated in the synthesis of two new ligands that feature S- and P-atom donors from sodium thiophenolate and potassium diphenylphosphide to yield an SNNS (5.2) and PNNP (5.3) extended pincer ligand, respectively. However, preliminary metalation conditions with 2.0 equiv. [Cu(NCMe)₄][NTf₂] suggest the coordination of only one copper center, as confirmed by multinuclear NMR spectroscopy and mass spectrometry analysis. However, this work provides a proof-of-concept for a versatile stepwise synthetic strategy for new 1,8-naphthyridine-based ligands whose bimetallic binding capabilities may be supported by expanding the scope of the metalation conditions.Chapter 6. The synthetic platforms and techniques developed in Chapters 2–5 and the insights garnered therein provided countless opportunities to further develop the homo- and heterobimetallic capabilities of dinucleating 1,8-naphthyridine-based ligands. This chapter presents the development of a new robust unsymmetrical 1,8-naphthyridine-based ligand, three preliminary heterobimetallic (Cu/Zn, Ni/Fe, and Cu/Mo) results, and one preliminary homobimetallic (Co/Co) result with relevance to CO₂ reduction systems and Pauson-Khand catalysis, respectively. Since these stories are works in progress, only a few highlights are presented.