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Metal–Dithiolene Bonding Contributions to Pyranopterin Molybdenum Enzyme Reactivity

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Metal–Dithiolene Bonding Contributions to Pyranopterin Molybdenum Enzyme Reactivity inorganics Review Metal–Dithiolene Bonding Contributions to Pyranopterin Molybdenum Enzyme Reactivity Jing Yang 1 , John H. Enemark 2 and Martin L. Kirk 1,* 1 Department of Chemistry and Chemical Biology, The University of New Mexico, MSC03 2060, Albuquerque, NM 87131-0001, USA; [email protected] 2 Department of Chemistry Biochemistry, University of Arizona, Tucson, AZ 85721, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-505-277-5992 Received: 2 February 2020; Accepted: 2 March 2020; Published: 5 March 2020 Abstract: Here we highlight past work on metal–dithiolene interactions and how the unique electronic structure of the metal–dithiolene unit contributes to both the oxidative and reductive half reactions in pyranopterin molybdenum and tungsten enzymes. The metallodithiolene electronic structures detailed here were interrogated using multiple ground and excited state spectroscopic probes on the enzymes and their small molecule analogs. The spectroscopic results have been interpreted in the context of bonding and spectroscopic calculations, and the pseudo-Jahn–Teller effect. The dithiolene is a unique ligand with respect to its redox active nature, electronic synergy with the pyranopterin component of the molybdenum cofactor, and the ability to undergo chelate ring distortions that control covalency, reduction potential, and reactivity in pyranopterin molybdenum and tungsten enzymes. Keywords: metal–dithiolene; pyranopterin molybdenum enzymes; fold-angle; tungsten enzymes; electronic structure; pseudo-Jahn–Teller effect; thione; molybdenum cofactor; Moco 1. Introduction It is now well-established that all known molybdenum-containing enzymes [1–3], with the sole exception of nitrogenase, contain a common pyranopterin dithiolene (PDT) (Figure1) organic cofactor (originally called molybdopterin (MPT)), which coordinates to the Mo center of the enzymes through the sulfur atoms of the dithiolene fragment. To date, the PDT component [4] of the molybdenum cofactor (Moco) is the only known occurrence of dithiolene ligation in biological systems. This cofactor is also found in anaerobic tungsten enzymes, and it may be one of the most ancient cofactors in biology [5]. The study of metal–dithiolene compounds (metallodithiolenes) has undergone a recent renaissance, with their synthesis, geometric structure, spectroscopy, bonding, and electronic structure having been recently highlighted [4,6–20]. Here, we briefly review the discovery of metallodithiolene compounds [13,21]. This history is followed by a more extensive discussion of key investigations into the myriad roles of the dithiolene ligands in the structure, bonding and reactivity of metal compounds, using multiple spectroscopic techniques, as well as theoretical calculations. Throughout this review, the key implications of these results for Mo and W enzymes are discussed. Inorganics 2020, 8, 19; doi:10.3390/inorganics8030019 www.mdpi.com/journal/inorganics Inorganics 2020, 8, 19 2 of 14 Inorganics 2020, 8, 19 2 of 14 Inorganics 2020, 8, 19 2 of 14 Figure 1. The reduced tetrahydro form of the pyranopterin dithiolene (PDT) coordinated to Mo in the molybdenum cofactor (Moco). In the enzymes, the Mo ion can redox cycle between the MoIV, MoV, and MoVI oxidation states. Figure 1. TheFigure reduced 1. The tetrahydro reduced tetrahydro form form of the of the pyranopterin pyranopterin dithiolene dithiolene (PDT) (PDT) coordinated coordinated to Mo in the to Mo in the molybdenum cofactor (Moco). In the enzymes, the Mo ion can redox cycle between the MoIV, MoV, In the early 1960s, several research groups reported intensely colored square planarIV V metal molybdenumand cofactor MoVI oxidation (Moco). states. In the enzymes, the Mo ion can redox cycle between the Mo , Mo , complexesand Mo VIwith oxidation chelating states. sulfur-donor ligands that could stabilize metal compounds in a range of formal oxidationIn thestates early related 1960s, several by one-electron research groups oxidation-reduction reported intensely colored (i.e., square redox) planar reactions metal (Figure 2) complexes with chelating sulfur-donor ligands that could stabilize metal compounds in a range of formal [22–24].In the McCleverty oxidationearly 1960s, states gave relatedseveral these by one-electronnovelresearch ligands oxidation-reductiongroups the reportedgeneral (i.e.,name intensely redox) “dithiolene” reactions colored (Figure insquare 2order)[ 22– 24 planarto]. emphasize metal complexestheir delocalized withMcCleverty chelating electronic gave thesesulfur-donor structures novel ligands [25]. ligands the These general that ligands name could “dithiolene” are stabilize also described in metal order tocompounds emphasizeas being their“non-innocent” in a range of formaldue to oxidationthe participationdelocalized states electronic related of the structures dithioleneby one-electron [25]. Theseligands ligands oxidation-reduction in arethe also multiple described one-electron as being(i.e., “non-innocent”redox) reactions reactions due of their(Figure metal 2) complexes toand the the participation inability of theto dithioleneassign a ligandsspecific in theoxidation multiple one-electronstate to the reactions metal of ion their or metal the dithiolene [22–24]. McClevertycomplexes andgave the these inability novel to assign ligands a specific the oxidationgeneral statename to the“dithiolene” metal ion or thein order dithiolene to emphasize theirligands delocalized [11].ligands electronic [11]. structures [25]. These ligands are also described as being “non-innocent” due to the participation of the dithiolene ligands in the multiple one-electron reactions of their metal complexes and the inability to assign a specific oxidation state to the metal ion or the dithiolene ligands [11]. Figure 2. Square planer metallodithiolene complexes. R = CN, CH3, Ph, CF3. Figure 2. Square planer metallodithiolene complexes. R = CN, CH3, Ph, CF3. Importantly, these non-innocent dithiolene ligands can modulate the nature of the covalent bonding with transition metal ions via the various redox states accessible to the dithiolene (Figure3)[ 13]. Importantly,The ene-1,2-dithiolate these non-innocent is the reduced dithiolene form of the ligandligands and can possesses modulate six π-electrons. the nature This ligand of the covalent form is both a σ-donor and π-donor that usually forms strong covalent bonds with an oxidized bonding with transitionFigure 2. Squaremetal ionsplaner via metallodithiolene the various redox complexes. states accessible R = CN, CH to3, thePh, dithioleneCF3. (Figure 3) [13]. The ene-1,2-dithiolatetransition metal ion, is as the is observedreduced in form the active of the sites ligand of most and pyranopterin possesses Mo six and π-electrons. W enzymes This ligand (e.g., Mo(V)/Mo(VI)-dithiolene bonds). The radical anion form with five π-electrons is usually formImportantly, is bothfound a σ in -donorthese molecules non-innocent and chelated π-donor by multiple dithiolenethat dithioleneusually liga ligands,formsnds can wherestrong modulate extended covalent delocalizationthe bonds nature with ofof the the an covalentoxidized bondingtransition with metalπ-electrons transition ion, and as mixed-valency ismetal observed ions assistsvia in thethe in theactivevarious stabilization sites redox of of most thestates metal–ligand pyranopterin accessible bonds. to TheMo the fully anddithiolene oxidized W enzymes (Figure (e.g., 3) [13].Mo(V)/Mo(VI)-dithiolene The ene-1,2-dithiolate1,2-dithione form of bonds).is the the ligand redu The possessesced radical form only of fouranion theπ-electrons ligand form andandwith can possesses five be described π-electrons six by π two-electrons. resonanceis usually This found ligand in structures (e.g., the 1,2-dithione and 1,2-dithiete). The low-lying empty π* orbitals of the S=C bonds formmolecules is both chelatedin thea σ dithione-donor by multiple can and accept π-donor πdithiolene-electron that density ligands,usually from electron-rich whereforms extendedstrong low-valent covalent delocalization transition bonds metals withof [16 ,the17 ],an π -electronsoxidized transitionand mixed-valency metalthereby ion, stabilizing asassists is observed such in compounds.the instabilization the However, active sitesof dithione-containing the of metal–ligandmost pyranopterin low-valent bonds. metal Mo The complexes and fully W enzymes are oxidized (e.g., 1,2- Mo(V)/Mo(VI)-dithiolenedithione formencountered of the muchligand lessbonds). possesses frequently The than onlyradical high-valent four anion π transition-electrons form metalwith and ions five can coordinated π be-electrons described by reduced is byusually forms two resonancefound in of dithiolene ligands. moleculesstructures chelated(e.g., the by 1,2-dithione multiple dithiolene and 1,2-dithiete). ligands, Thewhere low-lying extended empty delocalization π* orbitals of of the the π S=C-electrons bonds andin the mixed-valency dithione can acceptassists πin-electron the stabilization density from of the electron-rich metal–ligand low-valent bonds. transitionThe fully metalsoxidized [16,17], 1,2- dithionethereby stabilizingform of the such ligand compounds. possesses However,only four πdi-electronsthione-containing and can low-valentbe described metal by two complexes resonance are structuresencountered (e.g., much the 1,2-dithioneless frequently and than1,2-dithiete). high-valent The transitionlow-lying metalempty ions π* orbitals coordinated of the byS=C reduced bonds informs the dithione of dithiolene can
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