Synthesis, Structure and Reactivity of Homo- and Heterobimetallic First-Row Transition Metal Complexes Supported by Rigid Naphthyridine-Based Ligands

Chapter 1. A dicopper complex featuring a symmetrically bridging nitrile ligand and supported by a binucleating naphthyridine-based ligand, [Cu2(μ-η1:η1-MeCN)(DPFN)](NTf2)2, was treated with phosphaalkynes (RC≡P, isoelectronic analogues of nitriles) to yield dicopper complexes that exhibit phosphaalkynes in rare μ-η2:η2 binding coordination modes. X-ray crystallography revealed that these unusual “tilted” structures exist in two isomeric forms (R “up” vs. R “sideways”), depending on the steric profile of the phosphaalkyne’s alkyl group (R = Me, Ad, or tBu). Only one isomer is observed in both solution and the solid state for R = Me (sideways) and tBu (up). With intermediate steric bulk (R = Ad), the energy difference between the two geometries is small enough that both are observed in solution: NMR spectroscopy and computations indicate that the solid-state structure corresponds to the minor isomer observed in solution. Meanwhile, treatment of [Cu2(μ-η1:η1-MeCN)(DPFN)](NTf2)2 with 2-butyne affords [Cu2(μ-η2:η2-(MeC≡CMe))(DPFN)](NTf2)2: its similar ligand geometry demonstrates that the tilted μ-η2:η2 binding mode is not limited to phosphaalkynes but reflects a more general trend, which can be rationalized via an NBO analysis showing maximization of π-backbonding./nChapter 2. A bridging MeCN ligand in a dicopper(I) complex, [Cu2(μ-η1:η1-MeCN)(DPFN)](X)2 (X = weakly coordinating anions), is readily displaced by white phosphorus (P4) or yellow arsenic (As4). The X-ray structures of the resulting complexes provide the first examples of P4 and As4 binding to a bimetallic core through two adjacent edges, and the coordination of intact P4 and As4 tetrahedra was confirmed experimentally and computationally. These complexes react with N-heterocyclic carbenes (NHCs), and displacement of intact E4 (E = P, As) is observed upon reaction with a small NHC, tetramethylcarbene. For an NHC with a larger steric profile, IPr, an unusual nucleophile-triggered opening of P4 to a butterfly geometry is observed./nChapter 3. Metal-metal cooperation is integral to the function of many enzymes and materials, and model complexes hold enormous potential for providing insights into the capabilities of analogous multimetallic cores. However, the selective synthesis of heterobimetallic complexes still presents a significant challenge, especially for systems that hold the metals in close proximity and feature open or reactive coordination sites for both metals. To address this issue, a rigid, naphthyridine-based dinucleating ligand featuring distinct binding environments was synthesized. This ligand enables the selective synthesis of a series of MIICuI bimetallic complexes (M = Mn, Fe, Co, Ni, Cu, Zn), in which each metal center exclusively occupies its preferred binding pocket, from simple chloride salts. The precision of this selectivity is evident from cyclic voltammetry, ESI-MS and anomalous X-ray diffraction measurements./nChapter 4. Two unsymmetrical dinucleating naphthyridine-based ligands were employed to synthesize dicopper(I) chloride cores able to activate NaBPh4 to yield bridging phenyl organocopper complexes. These compounds display structural properties similar to those of related, reported symmetrical dicopper(I) complexes, but different electrochemical behavior. The unsymmetrical complexes activate aromatic and alkynyl C–H bonds. The resulting bridging alkynyl organocopper complexes show structural differences driven by the electron-donating or withdrawing character of the alkyne’s substituents and are active CuAAC catalysts./nChapter 5. Deconvoluting ligand effects from localized oxidation states in the spectroscopic signature of polynuclear systems remains challenging. A multiple wavelength anomalous X-ray diffraction (MAD) study was performed on a series of dicopper(I) and mixed-valent copper(I)-copper(II) complexes, supported by either symmetrical or unsymmetrical naphthyridine-based ligands. The MAD results revealed the extent of spin (de)localization for a range of complexes and were in excellent agreement with that observed by electron paramagnetic resonance spectroscopy. Both sets of spin localization results were corroborated by HF computations. Taken together, these results demonstrate and validate this technique’s utility for determining site-specific relative oxidation states in multimetallic systems.

Advisor: Tilley, T. Don