Richard A. Lewis

Metal templated design of protein interfaces

Metal coordination is a key structural and functional component of a large fraction of proteins. Given this dual role we considered the possibility that metal coordination may have played a templating role in the early evolution of protein folds and complexes. We describe here a rational design approach, Metal Templated Interface Redesign (MeTIR), that mimics the time course of a hypothetical evolutionary pathway for the formation of stable protein assemblies through an initial metal coordination event. Using a folded monomeric protein, cytochrome cb562, as a building block we show that its non-self-associating surface can be made self-associating through a minimal number of mutations that enable Zn coordination. The protein interfaces in the resulting Zn-directed, D2-symmetrical tetramer are subsequently redesigned, yielding unique protein architectures that self-assemble in the presence or absence of metals. Aside from its evolutionary implications, MeTIR provides a route to engineer de novo protein interfaces and metal coordination environments that can be tuned through the extensive noncovalent bonding interactions in these interfaces.

Metal-mediated self-assembly of protein superstructures: influence of secondary interactions on protein oligomerization and aggregation.

We have previously demonstrated that non-self-associating protein building blocks can oligomerize to form discrete supramolecular assemblies under the control of metal coordination. We show here that secondary interactions (salt bridges and hydrogen bonds) can be critical in guiding the metal-induced self-assembly of proteins. Crystallographic and hydrodynamic measurements on appropriately engineered cytochrome cb562 variants pinpoint the importance of a single salt-bridging arginine side chain in determining whether the protein monomers form a discrete Zn-induced tetrameric complex or heterogeneous aggregates. The combined ability to direct PPIs through metal coordination and secondary interactions should provide the specificity required for the construction of complex protein superstructures and the selective control of cellular processes that involve protein-protein association reactions.

Tuning the Reactivity of TEMPO by Coordination to a Lewis Acid: Isolation and Reactivity of MCl3(η1-TEMPO) (M = Fe, Al)

Addition of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) to MCl(3) (M = Fe, Al) results in the formation of MCl(3)(η(1)-TEMPO) [M = Fe (1), Al (2)]. Both 1 and 2 oxidize alcohols to generate ketones or aldehydes along with the reduced complexes MCl(3)(η(1)-TEMPOH) [M = Fe (3), Al (4)]. Complexes 1-4 were fully characterized, including analysis by X-ray crystallography. Additionally, control experiments indicated that neither MCl(3) (M = Al, Fe) nor TEMPO are capable of effecting the oxidation of alcohols independently.

Control of protein oligomerization symmetry by metal coordination: C2 and C3 symmetrical assemblies through Cu(II) and Ni(II) coordination.

We describe the metal-dependent self-assembly of symmetrical protein homooligomers from protein building blocks that feature appropriately engineered metal-chelating motifs on their surfaces. Crystallographic studies indicate that the same four-helix-bundle protein construct, MBPC-1, can self-assemble into C(2) and C(3) symmetrical assemblies dictated by Cu(II) and Ni(II) coordination, respectively. The symmetry inherent in metal coordination can thus be directly applied to biological self-assembly.

Synthesis and Characterization of an Iron(IV) Ketimide Complex

Addition of 4 equiv of LiN═C(t)Bu(2) to FeCl(2) in Et(2)O/THF results in the formation of [Li(THF)](2)[Fe(N═C(t)Bu(2))(4)] (1). Oxidation of 1 with 0.5 equiv of I(2) in Et(2)O/DME yields [Li(DME)][Fe(N═C(t)Bu(2))(4)] (2) in moderate yield. Both 1 and 2 are high spin and exhibit tetrahedral geometries in the solid state. Oxidation of 1 with 1 equiv of I(2) in Et(2)O yields Fe(N═C(t)Bu(2))(4) (3) in good yield. Surprisingly, complex 3 exhibits a diamagnetic ground state and a nearly square planar geometry about the Fe center.

Stabilizing high-valent metal ions with a ketimide ligand set: synthesis of Mn(N=C(t)Bu2)4.

Reaction of MnCl(2) with 4 equiv of Li(N=C(t)Bu(2)) generates [Li(THF)](2)[Mn(N=C(t)Bu(2))(4)] (1) in 80% yield. Oxidation of 1 with 0.5 equiv of I(2) produces [Li][Mn(N=C(t)Bu(2))(4)] (2) in 88% yield. Both complexes 1 and 2 exhibit tetrahedral structures about the Mn center in the solid-state, as determined by X-ray crystallography. Reaction of 2 with 12-crown-4 generates [Li(12-crown-4)(2)][Mn(N=C(t)Bu(2))(4)] (3) in 94% yield. Interestingly, in the solid-state, complex 3 exhibits a squashed tetrahedral structure about Mn. Addition of 1 equiv of I(2) to 1 generates the Mn(IV) ketimide, Mn(N=C(t)Bu(2))(4) (4), in 75% yield. Complex 4 was fully characterized, including analysis by X-ray crystallography and cyclic voltammetry. Like 3, complex 4 also exhibits a squashed tetrahedral structure in the solid-state. Interestingly, thermolysis of complex 4 at 50 °C for 6 h results in the formation of Mn(3)(N=C(t)Bu(2))(6) (6), which can be isolated in 49% yield. Also observed in the reaction mixture is pivalonitrile, isobutylene, and isobutene, the products of ketimide ligand oxidation. We have also synthesized the homoleptic Cr(IV) ketimide complex, Cr(N=C(t)Bu(2))(4) (5), and have analyzed its electrochemical properties with cyclic voltammetry.

Reactivity and Mössbauer Spectroscopic Characterization of an Fe(IV) Ketimide Complex and Reinvestigation of an Fe(IV) Norbornyl Complex

Thermolysis of Fe(N═C(t)Bu2)4 (1) for 8 h at 50 °C generates the mixed valent Fe(III)/Fe(II) bimetallic complex Fe2(N═C(t)Bu2)5 (2) in moderate yield. Also formed in this reaction are tert-butyl cyanide, isobutane, and isobutylene, the products of ketimide oxidation by the Fe(4+) center. Reaction of 1 with 1 equiv of acetylacetone affords the Fe(III) complex, Fe(N═C(t)Bu2)2(acac) (3), concomitant with formation of bis(tert-butyl)ketimine, tert-butyl cyanide, isobutane, and isobutylene. In addition, the Mössbauer spectra of 1 and its lower-valent analogues [Li(12-crown-4)2][Fe(N═C(t)Bu2)4] (5) and [Li(THF)]2[Fe(N═C(t)Bu2)4] (6) were recorded. We also revisited the chemistry of Fe(1-norbornyl)4 (4) to elucidate its solid-state molecular structure and determine its Mössbauer spectrum, for comparison with that recorded for 1.

Synthesis and Characterization of [M2(NCtBu2)5]− (M=Mn, Fe, Co): Metal Ketimide Complexes with Strong Metal–Metal Interactions†

The study of metal–metal interactions has provided many important insights into transition-metal bonding and electronic structure. This is perhaps best exemplified by the synthesis of quintuply bonded [Ar’CrCrAr’] (Ar’= C6H32,6(C6H3-2,6-iPr2)2), which has intrigued both experimentalists and theoreticians since it was first reported in 2005. Complexes with metal–metal bonds also exhibit interesting optoelectronic properties and intriguing chemical reactivity. Interestingly, a survey of complexes with metal–metal bonds shows a large knowledge gap between the late first-row transition metals and the rest of the transition-metal block. For example, the Cambridge Structural Database contains only a few M2 4+ complexes with metal– metal bonds for Mn (4 structures), Fe (28 structures), and Co (54 structures), whereas many more structure are known for Cr (> 500 structures), Ru (> 500), and Rh (> 1500 structures). These trends can be rationalized by the contracted nature of the 3d electrons for the later first-row transition metals, and highlights the challenge of making metal–metal bonds with these elements. In this regard, the development of new ligands that can promote metal–metal bonding would be of significant benefit for the exploration of these interactions and their application in the field of catalysis. Herein we demonstrate the ability of the ketimide ligand, [N=CR2] , to promote metal–metal interactions, specifically in the ketimide-bridged transition-metal complexes, [M2(N=CtBu2)5] (M = Mn, Fe, Co), which exhibit short metal–metal distances and strong inter-metal magnetic communication. Addition of 2.5 equiv of Li(N=CtBu2) to MCl2 (M = Mn, Fe, and Co) in THF, followed by addition of 1 equiv of [12]crown-4, provides [Li([12]crown-4)2][M2(N=CtBu2)5] (M = Mn, 1; Fe, 2 ; Co, 3) in 58–78% yield [Eq. (1)]. Complexes 1–3 crystallize in the monoclinic space group C2/c as discrete cation–anion pairs. Each complex features

Quantifying the Electron Donor and Acceptor Abilities of the Ketimide Ligands in M(N═CtBu2)4 (M = V, Nb, Ta)

Addition of 4 equiv of Li(N═C(t)Bu2) to VCl3 in THF, followed by addition of 0.5 equiv of I2, generates the homoleptic V(IV) ketimide complex, V(N═C(t)Bu2)4 (1), in 42% yield. Similarly, reaction of 4 equiv of Li(N═C(t)Bu2) with NbCl4(THF)2 in THF affords the homoleptic Nb(IV) ketimide complex, Nb(N═C(t)Bu2)4 (2), in 55% yield. Seeking to extend the series to the tantalum congener, a new Ta(IV) starting material, TaCl4(TMEDA) (3), was prepared via reduction of TaCl5 with Et3SiH, followed by addition of TMEDA. Reaction of 3 with 4 equiv of Li(N═C(t)Bu2) in THF results in the isolation of a Ta(V) ketimide complex, Ta(Cl)(N═C(t)Bu2)4 (5), which can be isolated in 32% yield. Reaction of 5 with Tl(OTf) yields Ta(OTf)(N═C(t)Bu2)4 (6) in 44% yield. Subsequent reduction of 6 with Cp*2Co in toluene generates the homoleptic Ta(IV) congener Ta(N═C(t)Bu2)4 (7), although the yields are poor. All three homoleptic group 5 ketimide complexes exhibit squashed tetrahedral geometries in the solid state, as determined by X-ray crystallography. This geometry leads to a d(x(2)-y(2))(1) ((2)B1 in D(2d)) ground state, as supported by DFT calculations. EPR spectroscopic analysis of 1 and 2, performed at X- and Q-band frequencies (∼9 and 35 GHz, respectively), further supports the (2)B1 ground-state assignment, whereas comparison of 1, 2, and 7 with related group 5 tetra(aryl), tetra(amido), and tetra(alkoxo) complexes shows a higher M-L covalency in the ketimide-metal interaction. In addition, a ligand field analysis of 1 and 2 demonstrates that the ketimide ligand is both a strong π-donor and strong π-acceptor, an unusual combination found in very few organometallic ligands.