William B. Tolman

Reaction of the sterically hindered α-ketocarboxylate 2,6-di(mesityl)benzoylformate (MesBF) with the iron(II) complexes LFeCl₂ [L = N,N,N′,N′-tetramethylpropylenediamine (Me₄pda) or 6,6′-dimethyl-2,2′-bipyridine (dmby)] yielded LFe(Cl)(MesBF) (1 or 2). X-ray crystal structures of these complexes showed that they closely model the active site structure of the nonheme iron halogenase enzyme SyrB2. A similar synthetic procedure using benzoylformate with L = dmby yielded (dmby)Fe[(O₂CC(O)Ph)]₂ (3) instead, demonstrating the need for the sterically hindered α-ketocarboxylate to assemble the halogenase model compounds. In order to make reactivity comparisons among the structurally related iron(II) complexes of benzoylformates of varying steric properties, the complexes [LFe(O₂CC(O)Ar)]_n (4−6) were prepared, where L′ = tris(pyridylmethyl)amine (tpa) and Ar = 2,6-dimesitylphenyl, 2,6-dip-tolylphenyl, or 2,4,6-trimethylphenyl, respectively. X-ray structures for the latter two cases (5 and 6) revealed dinuclear topologies (n = 2), but UV–vis and ¹H NMR spectroscopy indicated that all three complexes dissociated in varying degrees to monomers in CH₂Cl₂ solution. Although compounds 1−6 were oxidized by O₂, oxidative decarboxylation of the α-ketocarboxylate ligand(s) only occurred for 3. These results indicate that the steric hindrance useful for structural modeling of the halogenase active site prohibits functional mimicry of the enzyme.

A series of copper(I)−α-ketocarboxylate complexes have been prepared and shown to exhibit variable coordination modes of the α-ketocarboxylate ligand. Reaction with O₂ induces decarboxylation of this ligand, and the derived copper−oxygen intermediate(s) has been intercepted, resulting in hydroxylation of an arene substituent on the supporting N-donor ligand. Theoretical calculations have provided intriguing mechanistic notions for the process, notably implicating hydroxylation pathways that involve novel [Cu^I−OOC(O)R] and [Cu^II−O^-• ↔ Cu^IIIO^2-]⁺ species.

A general method for the preparation of multidentate ligands comprised of a multi-imine platform derived from 1,1,1-tris(aminomethyl)ethane or tris(aminoethyl)amine connected to bi- and tridentate N-donor chelates has been developed. The feasibility of the method has been demonstrated through the synthesis and characterization of a large set of these ligand types. Complexation to Cu(I) was accomplished for several cases, yielding tri- and tetracopper(I) complexes that have been characterized in solution by NMR spectroscopy and conductivity, and in the solid state by elemental analysis, mass spectrometry, and/or X-ray crystallography. These complexes are potentially useful for modeling multicopper protein active sites.

Heterobimetallic CuPd and CuPt bis(μ-oxo) complexes have been prepared by the reaction of (PPh₃)₂MO₂ (M = Pd, Pt) with LCu(I) precursors (L = β-diketiminate and di- and triamine ligands) and characterized by low-temperature UV−vis, resonance Raman, and ¹H and ³¹P{¹H} NMR spectroscopy in conjunction with DFT calculations. The complexes decompose upon warming to yield OPPh₃, and in one case this was shown to occur by an intramolecular process through crossover experiments using double-labeling (oxo and phosphine). The reactivity of one of the complexes, L^Me2Cu(μ-O)₂Pt(PPh₃)₂ (L^Me2 = β-diketiminate), with a variety of reagents including CO₂, 2,4-di-tert-butylphenol, 2,4-di-tert-butylphenolate, [NH₄][PF₆], and dihydroanthracene, was compared to that of homometallic Pt₂ and Cu₂ counterparts. Unlike typical [Cu₂(μ-O)₂]²⁺ cores which have electrophilic oxo groups, the oxo groups in the [Cu(μ-O)₂Pt]⁺ core behave as bases and nucleophiles, similar to previously described Pt₂ compounds. In addition, however, the [Cu(μ-O)₂Pt]⁺ core is capable of oxidatively coupling 2,4-di-tert-butylphenol and 2,4-di-tert-butylphenolate. Theoretical evaluation of the electron affinities, basicities, and H-atom transfer kinetics and thermodynamics of the Cu₂ and CuM (M = Pd, Pt) cores showed that the latter are more basic and form stronger O−H bonds.

Using interwoven experimental and theoretical methods, detailed studies of several structurally defined 1:1 Cu–O₂ complexes have provided important fundamental chemical information useful for understanding the nature of intermediates involved in aerobic oxidations in synthetic and enzymatic copper-mediated catalysis. In particular, these studies have shed new light on the factors that influence the mode of O₂ coordination (end-on vs side-on) and the electronic structure, which can vary between Cu(II)–superoxo and Cu(III)–peroxo extremes.

To assess supporting ligand effects on S−S bond activation, a series of [Cu₂(μ-η²:η²-S₂)]²⁺ complexes supported by various β-diketiminate or anilido-imine ligands (L) were synthesized via the reaction of Cu(I) precursors LCu(CH₃CN) with S₈. For the cases where L = β-diketiminate, the syntheses were complicated by formation of clusters [Cu(SR)]₄, where SR represents the ligand functionalized by sulfur at the central methine position. The [Cu₂(μ-η²:η²-S₂)]²⁺ products were characterized by X-ray crystallography and electronic absorption and resonance Raman spectroscopy. Correlations among the Cu−S, Cu−Cu, and S−S distances and the ν(S−S) values were observed and interpreted within the framework of a previously described bonding picture (Chen, P.; Fujisawa, K.; Helton, M. E.; Karlin, K. D.; Solomon, E. I. J. Am. Chem. Soc.2003, 125, 6394). Comparison of these data to those for other relevant species revealed a remarkable degree of S−S bond activation in the compounds supported by the β-diketiminate and anilido-imine ligands, which through strong electron donation increase backbonding from the copper ions into the S−S σ* orbital and cause S−S bond weakening. Reactions of one of the complexes supported by an anilido-imine ligand with PPh₃ and xylyl isocyanide were explored, revealing facile transfer of sulfur to PPh₃ but only displacement of sulfur to yield a LCu(I)−CNAr (Ar = xylyl) complex with the isocyanide.