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  • 1. www.elsevier.com/locate/ica Inorganica Chimica Acta 331 (2002) 246– 256 Remote ligand substituent effects on the properties of oxo-Mo(V) centers with a single ene-1,2-dithiolate ligand Frank E. Inscore, Hemant K. Joshi, Anne E. McElhaney, John H. Enemark * Department of Chemistry, Uni6ersity of Arizona, Tucson, AZ 85721, USA Received 25 August 2001; accepted 12 November 2001 Dedicated to Professor A.G. SkyesAbstract The oxomolybdenum mono-ene-1,2-dithiolate complex (Tp*)MoO(bdtCl2) (3) has been synthesized and characterized (Tp* ishydrotris(3,5-dimethyl-1-pyrazolyl)borate; bdtCl2 is 3,6-dichloro-1,2-benzenedithiolate). The X-ray structural data show that 3 ,crystallizes in the monoclinic space group, P21/c, where a= 7.963 (3), b=26.272 (11), c= 14.016 (6) A, i= 105.352 (7). The(Tp*)MoO(bdtCl2) molecule exhibits a distorted pseudo-octahedral coordination geometry, with the Mo atom ligated by aterminal oxo atom, two sulfur donor atoms of the bdtCl2 ligand and three nitrogen atoms of the tridentate facially coordinatedTp* ligand. The coordination environment about the Mo atom is similar to that of (Tp*)MoO(bdt) (1) (bdt is 1,2-benzenedithi-olate), but the fold angle between the MoS2 plane and S2C2 plane of the bdtCl2 ligand (q =6.9°) is substantially smaller than thefeature in 1 (q=21.3°). The similar IR, EPR, and electronic absorption spectroscopic results for 1 and 3 indicate that the electronwithdrawing nature of the chlorine substituents of 3 does not significantly perturb the electronic structure of the Mo(V) center.However, the solution redox potentials and the gas-phase ionization energies are sensitive to remote substituent effects. © 2002Elsevier Science B.V. All rights reserved.Keywords: Oxo-molybdenum ene-1,2-dithiolate complexes; Molybdenum dithiolate complexes; Pyranopterin molybdenum enzymes; Sulfite oxidase1. Introduction atom transfer [1]. During enzymatic turnover, the molybdenum center is proposed to shuttle through the The syntheses and physical characterization of new Mo(VI)/Mo(V)/Mo(IV) oxidation states [1]. The X-raymono-oxo molybdenum complexes possessing various crystal structures are now known for a large number ofene-1,2-dithiolate ligands coordinated to the metal cen- these enzymes, and all reveal a common active siteter continues to be an important area of study in structural feature, which consists of at least one pyra-connection with the pyranopterin molybdenum en- nopterin-ene-1,2-dithiolate (Fig. 1) coordinated to thezymes. These molybdenum-containing enzymes, which molybdenum center [2–15]. At least one terminal oxoare categorized into three families based on structure ligand is associated with the Mo active sites duringand reactivity (xanthine oxidase, DMSO reductase, and catalysis. The role of the pyranopterin ene-1,2-dithio-sulfite oxidase), catalyze a variety of specific two-elec- late has been postulated to couple the Mo center intotron redox reactions that are coupled to formal oxygen efficient superexchange pathways for facilitating elec- tron-transfer regeneration of the active site [16] and to Abbre6iations: bdt, 1,2-benzenedithiolate; bdtCl2, 3,6-dichloroben- modulate the Mo center reduction potential [1,16,17].zenedithiolate; qdt, 2,3-dithioquinoxaline; tdt, 3,4-toluenedithiolate; The structural relevance and importance of the coor-Tp*, hydrotris(3,5-dimethyl-1-pyrazolyl)borate; (S S), generic ene- dinated pyranopterin-ene-1,2-dithiolate on the reactiv-1,2-dithiolate ligand that forms a five-membered chelate ring with the ity of these Mo containing enzymes have provided theMo atom. * Corresponding author. Tel.: + 1-520-621 2245; fax: + 1-520-521 motivation to synthesize and characterize new oxo-Mo8407. dithiolate complexes. The series of complexes shown in E-mail address: jenemark@u.arizona.edu (J.H. Enemark). Fig. 2 possess a {(Tp*)MoVO}2 + center, which pro-0020-1693/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 0 - 1 6 9 3 ( 0 1 ) 0 0 8 1 7 - 9
  • 2. F.E. Inscore et al. / Inorganica Chimica Acta 331 (2002) 246–256 247vides an invariant structural unit that allows the funda- 2. Experimentalmental properties of oxo-Mo complexes with an ene-1,2-dithiolate ligand to be investigated. These 2.1. General methods and materialsmono-oxo-Mo compounds represent minimal structuralmodels of the Mo(V) active site in the sulfite oxidase All reactions, synthetic operations and manipulationsfamily, which possess a single pyranopterin ene-1,2- followed strict anaerobic procedures and were per-dithiolate coordinated to the Mo center throughout the formed under a dry blanket of pre-purified argon gascatalytic cycle. The spectroscopic silence of the (Tp*) using standard Schlenk techniques, a high-vacuum/gasligand has allowed for detailed electronic structure double line manifold, and an inert atmospheric glovestudies to be initiated, which have defined the nature of bag. Synthetic operations were also carried out in anoxo-Mo-S (dithiolate) interactions in this system [16– inert atmosphere glove box filled with pure dinitrogen24]. Of particular importance, are the spectroscopic gas. The argon was predried by passing the high-purity-studies of (Tp*)MoO(bdt) (1) and (Tp*)MoO(tdt) (2) grade inert gas through a series of drying towers.that have provided insight into oxygen atom transfer Dinitrogen was obtained directly from a pressurizedreactivity and electron-transfer regeneration of the ac- liquid nitrogen cryogenic transfer/storage dewar. Alltive site in the pyranopterin Mo enzymes [16,19]. Subse- glassware was oven dried at 150 °C and Schlenk warequent studies on (Tp*)MoO(qdt) (4) have revealed that was further purged by repeated evacuation and inertthe nature of the ene-1,2-dithiolate ligand plays an gas flushes prior to use. Tetrahydrofuran (THF) andimportant role in defining the spectroscopic differences toluene were distilled from Na/benzophenone; triethy-observed within this series [17,23]. A continuing area of lamine was distilled from Na/K amalgam [25]. Theimportance relates to the electron-donating ability of prepurified solvents were subsequently transferred andthe ene-1,2-dithiolate, and how this affects the geomet- stored under N2 over fresh drying agents. These sol-ric and electronic structures of oxo-Mo systems possess- vents were freshly distilled under nitrogen prior to use,ing mono-ene-1,2-dithiolate coordination. thoroughly degassed by repeated freeze–thaw–pump We describe here the syntheses and physical charac- cycles, and transferred to reaction vessels via steelterization of (3) [hydrotris(3,5-dimethyl-1-pyrazolyl)- cannulae techniques under a positive pressure of inertborato](3,6-dichloro-benzene-1,2-dithiolato)-oxomolyb gas. Dichloromethane, 1,2-dichloroethane, cyclohexene,denum(V), a new oxo-Mo(V) mono-ene-1,2-dithiolate toluene (EM Science, Omnisolv), n-hexane and n-pen-complex that has electron withdrawing groups on the tane (Burdick and Jackson) were used as received andaromatic ring. The X-ray crystal structure, electrochem- deoxygenated by argon saturation prior to use. Solventsical behavior, and spectroscopic data for 3 are com- employed in the spectroscopic characterization studiespared with those for related (Tp*)MoO(S S) complexes were degassed by freeze–thaw– pump cycling before(defined in Fig. 2). use. The 1,2-dichloroethane used in the electrochemical studies was of anhydrous grade (EM Science; Drisolv) and required no further purification. Reagents were generally used as received. Molybdenum pentachloride (MoCl5, Aldrich) was dried in vacuo and stored under dinitrogen prior to use. Potassium hydrotris(3,5- dimethyl-1-pyrazolyl)borate (KTp*) and the precursorFig. 1. Structure of the pyranopterin derived from protein crystallo- complex, (Tp*)MoVOCl2, were prepared according tographic studies (shown in fully reduced form) [2 –15]. literature procedures [18]. The ligands H2bdt (1,2-ben- zenedithiol) and H2bdtCl2 (3,6-dichloro-1,2-ben- zenedithiol) employed in the syntheses of the (Tp*)MoVO(S S) compounds (1, 3) were used as re- ceived from Aldrich. The preparation of (Tp*)Mo- O(bdt) (1) followed from published procedures [20,21]. The synthesis, isolation, purification and characteriza- tion of (Tp*)MoO(bdtCl2) (3) is described below. 2.2. Preparation of compounds Highly purified (Tp*)MoOCl2 (500 mg, 1.0 mmol) was added to an evacuated Schlenk flask and dissolvedFig. 2. Stereochemistry of the {(Tp*)MoVO}2 + system containing in 50 ml of dry degassed toluene. The mixture wasequatorial ene-1,2-dithiolate ligands (S S) coordinated to the Mo deoxygenated thoroughly with argon saturation whilecenter. Structures of the dithiolate dianions (S S) are shown. being stirred at 80 °C. Solid H2bdtCl2 (220 mg, 1.1
  • 3. 248 F.E. Inscore et al. / Inorganica Chimica Acta 331 (2002) 246–256Table 1 n-pentane. The red–brown powder precipitate was col-Crystal data and structure refinement parameters for lected by filtration and washed with n-pentane until the(Tp*)MoO(bdtCl2) (3) eluant was clear. The powder was then dissolved inEmpirical formula C21H24BCl2MoN6OS2·1/2C6H14 dichloromethane, filtered to remove any insoluble mate-Formula weight 661.33 rials, and evaporated to dryness in vacuo. The solid wasTemperature (K) 173(2) pumped on for several hours to ensure dryness and theWavelength (A) , 0.71073 complete removal of excess triethylamine (Et3N). TheCrystal system MonoclinicSpace group P2(2)/c solid material was re-dissolved in dichloromethane,Unit cell dimensions concentrated, and loaded on a silica gel chromato- a (A) , 7.963(3) graphic column (230–400 mesh, pore diameter 60 A, , b (A) , 26.272(11) Merck) under a positive pressure of argon. A red– , c (A) 14.016(6) brown fraction (band c 2) eluted off the column using h (°) 90 i (°) 105.352(7) dichloromethane–cyclohexene (1:3) as the eluant. The k (°) 90 purity of (Tp*)MoO(bdtCl2) was confirmed by TLC ,V (A3) 2828(2) analysis. The red–brown powder was evaporated toZ 4 dryness in vacuo. The compound was re-dissolved inDcalc (Mg m−3) 1.553 dichloromethane, and layered with n-pentane to yield aAbsorption coefficient (mm−1) 0.83F(000) 1252 dark red–brown crystalline material. The crystallineCrystal size (mm) 0.25×0.10×0.03 material was filtered, washed and then dried in vacuum.Theta range for utilized data (°)1.69–27.78 The product was characterized by IR, UV–Vis, EPRLimiting indices −105h59, −345k534, and mass spectroscopy. Suitable crystals for X-ray dif- −185l517 fraction studies were obtained by a slow diffusion ofReflections utilized 30 776Independent reflections 6266 [Rint = 0.0539] n-pentane (or n-hexane) into a concentratedCompleteness to theta =27.78° 93.7% dichloromethane solution of purified 3.Absorption correction NoneMax/min transmission 0.9757, 0.8203 2.3. X-ray crystal structure determinationsRefinement method Full-matrix least-squares on F 2Data/restraints/parameters 6266/0/340Goodness-of-fit on F 2 1.040 A burgundy plate of 3 was mounted on a glass fiberFinal R indices [I2sigma(I)] R1 = 0.0442, wR2 = 0.1005 for structure determination using a Bruker SMARTR indices (all data) R1 = 0.0764, wR2 = 0.1147 1000 CCD detector X-ray diffractometer. Key parame-Largest difference peak and hole 0.636 and −0.432 ters for the structure determination are summarized in , (e A−3) Table 1. Empirical absorption and decay corrections ,RMS difference density (e A−3) 0.098 were applied using the program SADABS. The structure was solved by direct methods using SHELXS in themmol) was added in slight excess to the suspension Bruker SHELXTL (Version 5.0) software package. Refin-under a positive pressure of argon. The resulting solu- ements were performed using SHELXL and illustrations were made using XP. Hydrogen atoms were added attion was purged with argon for 20 min. Dry degassed idealized positions, constrained to ride on the atom toEt3N (0.40 ml, 2.8 mmol) was added slowly dropwise which they are bonded and given thermal parametersvia gas tight syringe to this rigorously stirring solution. equal to 1.2 or 1.5 times Uiso of that bonded atom. AThe mildly refluxing reaction solution was observed to parameter describing extinction was included. Scatter-change gradually from an emerald green to a dark ing factors and anomalous dispersion were taken fromred – brown color after 4 h of stirring. The reaction International Tables Vol C, Tables 4.2.6.8 and 6.1.1.4progress, and hence optimal yield, was monitored by [26]. The crystals contain one half molecule solventTLC analysis (silica gel 60 F254 plastic sheets, EM (hexane) of crystallization per asymmetric unit (TableScience). The reaction was stopped upon observing the 1). The large values of thermal displacement parametersnear disappearance of the green (Tp*)MoOCl2 precur- of the carbon atoms of the hexane of crystallization cansor concomitant with the maximal formation of the be attributed to the partial loss of solvent moleculesred – brown product. Upon completion of the reaction, during the time interval between crystal isolation andthe blue– green precipitate, primarily Et3N·HCl result- low temperature data collection.ing from the hydrogen abstraction and ligand exchangeprocesses, was filtered off the hot solution under dry 2.4. Other physical measurementsargon. The filtrate was cooled to room temperature(r.t.) and evaporated to dryness with a rotorary evapo- Mass spectra were recorded on a JEOL HX110 high-rator. The solid red–brown residue was re-dissolved in resolution sector instrument utilizing fast atom bom-toluene, concentrated under vacuum, and layered with bardment (FAB) ionization in a matrix of 3-nitrobenzyl
  • 4. F.E. Inscore et al. / Inorganica Chimica Acta 331 (2002) 246–256 249alcohol (NBA). Infrared (IR) vibrational spectroscopic 0.020–0.023 eV. All data were intensity corrected withdata were collected on a Nicolet Avatar ESP 360 FT-IR an experimentally determined instrument analyzer sen-spectrophotometer. The IR spectra (4000– 400 cm − 1) sitivity function. The He I spectra were corrected forwere measured in KBr disks or as dichloromethane the presence of ionizations from other lines (He Ib line,solutions (between NaCl plates) at r.t. Electronic ab- 1.9 eV higher in energy and 3% the intensity of the Hesorption spectra of samples solvated in 1,2- Ia line). All samples sublimed cleanly with no de-dichloroethane solutions were recorded with a 1-cm tectable evidence of decomposition products in the gaspathlength Helma quartz cell equipped with a Teflon phase or as a solid residue. The sublimation tempera-stopper, on a modified Cary 14 (with OLIS interface, tures were monitored using a ‘‘K’’ type thermocouple250 – 2600 nm) spectrophotometer. Quantitative absorp- passed through a vacuum feed and attached directly totion spectra were acquired at 2.0 nm resolution using a the aluminum ionization sample cell. The sublimationdual-beam Hitachi U-3501 UV– Vis – NIR spectropho- temperatures (in °C, 10 − 4 Torr) were as follows:tometer calibrated with known mercury lines and a 6% (Tp*)MoO(bdtCl2), 198 °C; and (Tp*)MoO(bdt),neodymium doped laser glass standard (Schott Glass). 183 °C.Absorption spectra were analyzed using Hitachi sup-plied Grams software. Electron paramagnetic resonance(EPR) spectra at X-band frequency ( 9.1 GHz) of 3. Results and discussionsolution (298 K) and frozen glasses (77 K) were ob-tained on a Bruker ESP 300 spectrometer. The EPR 3.1. Syntheses, properties and physical characterizationsamples were prepared as 1.0 or 2.0 mM solutions in of (Tp*)MoO(bdtCl2)dry degassed toluene. Cyclic voltammetric (CV) datawere collected on a Bioanalytical Systems (BAS) CV-50 The synthesis of (Tp*)MoO(bdtCl2) was achieved byW system. BAS supplied software provided scan acqui- a ligand exchange reaction between the precursor com-sition control and data analysis/graphics capabilities. plex (Tp*)MoOCl2 and free ligand H2bdtCl2 in theThe electrochemical cell employed was based on a presence of a strong base (Et3N), as with other relatednormal three-electrode configuration. This cell consists compounds [16,18,20,21,27,28]. The identity of the re-of a platinum disk working electrode (1.6 mm diameter, action product was confirmed by its high resolutionBAS), a platinum wire counter electrode (BAS) and a mass spectrum, which shows an [M+H]+ peak thatNaCl saturated Ag/AgCl reference electrode (BAS). gives m/z= 619.0063 (calculated, 619.0059) and corre-Prior to each experiment, the electrode was polished sponds to the formula [12C21H25N11B32S35Cl2O97Mo]. 6 2using 0.05 mm alumina (Buehler) and electrochemically The product is soluble in dichloromethane,cleaned in dilute sulfuric acid. Cyclic voltammetric dichloroethane, toluene and benzene. This compoundmeasurements of (Tp*)MoO(bdtCl2) and related appeared to be relatively stable in air; however, to(Tp*)MoO(S S) complexes were performed in dry de- ensure structural integrity and sample purity, thegassed 1,2-dichloroethane solutions (10 ml, 1 mM, product was stored under argon prior to use.25 °C) over a potential window of 91.5 V versus The solid-state IR spectrum of (Tp*)MoO(bdtCl2) inAg/AgCl with 0.1–0.2 M dried tetra-n-butylammonium KBr exhibited bands characteristic of the (Tp*) ligandtetrafluoroborate [n-Bu4N][BF4] (Aldrich) as the sup- (w(B H) =2554 cm − 1) as well as a strong absorption atporting electrolyte. The background scans of dry/de- 933 cm − 1 typical of {MoVO}3 + containing speciesoxygenated DCE with the [n-Bu4N][BF4] supporting [16,18]. The solution IR spectrum of (Tp*)MoO(bdtCl2)electrolyte exhibited no electroactive impurities or sol- in dichloromethane showed no significant frequencyvent decomposition within the potential window em- shifts or additional bands, indicating structural in-ployed. Ferrocene was utilized as an internal standard, tegrity was maintained upon solvation. The absence ofand all potentials were referenced relative to the Fc/ a strong absorption at 961 cm − 1 associated with theFc+ couple. He I gas-phase photoelectron spectra w(Mo O) stretching mode of the (Tp*)MoOCl2 precur-(PES) were collected on a spectrometer with a 36-cm sor was indicative of the relative purity of theradius hemispherical analyzer (8 cm gap, McPherson), (Tp*)MoO(bdtCl2) compound. The frequencies for thesample cells, excitation sources, and detection and con- Mo O vibration in the (Tp*)MoO(S S) complexes con-trol electronics using methods that have been previously taining a monoene-1,2-dithiolate ligand span a narrowdescribed in detail [22]. The absolute ionization energy range (926–941 cm − 1), and are lower than that ob-scale for the He I experiments was calibrated by using served for the (Tp*)MoOCl2 precursor [16,18,20]. Thethe 2E1/2 ionization of methyl iodide (9.538 eV), with lower w(Mo O) observed in the structurally character-the argon 2P3/2 ionization (15.759 eV) used as an inter- ized series, (Tp*)MoO(bdt) (931 cm − 1)B −1nal calibration lock during the experiment. During data (Tp*)MoO(bdtCl2) (933 cm )B (Tp*)MoO(qdt) (941collection the instrument resolution (measured using cm − 1) relative to related halide complexes [18,29,30]the FWHM of the argon 2P3/2 ionization peak) was can be attributed to the weaker p-donor properties of
  • 5. 250 F.E. Inscore et al. / Inorganica Chimica Acta 331 (2002) 246–256Table 2Summary of ground-state IR a and EPR b spectral data for (Tp*)MoO(S S)Complex w(Mo O) g1 g2 g3 Žg c ŽA d Reference(1) (Tp*)MoO(bdt) 931 2.004 1.972 1.934 1.971 37.0 [20,21](2) (Tp*)MoO(tdt) 926 2.004 1.974 1.937 1.972 34.3 [18](3) (Tp*)MoO(bdtCl2) 933 2.002 1.974 1.938 1.973 35.6 This work(4) (Tp*)MoO(qdt) 941 2.005 1.975 1.952 1.975 35.6 [34] a IR obtained in KBr disks (cm−1). b X-band frequencies, in toluene, anisotropic gi at 77 K. c Isotropic Žg values at 298 K. d 95,97 Mo, ×10−4 cm−1, isotropic ŽA values at 298 K.the halide atom versus the dithiolate donor ligands. Table 3 presents selected interatomic distances andThis trend in w(Mo O) stretching frequencies reflecting bond angles. The molecule exhibits a distorted pseudo-the stronger p-donor ability of equatorial sulfur donor octahedral coordination geometry, where the Mo atomatoms has been observed between a variety of is ligated by a terminal oxo atom, two sulfur donoranalogous thiolate and halide oxo-Mo systems [27,31]. atoms of the bdtCl2 ligand and three nitrogen atoms of The X-band CW EPR spectra of (Tp*)MoVO- the tridentate facially coordinated Tp* ligand. No sym-(bdtCl2), measured in both frozen (77 K) and fluid (298 metry is imposed on 3 by this space group, but theK) toluene solutions, show that only one EPR-active effective symmetry is Cs. The terminal oxo ligand andmolybdenum species is present. The fluid solution EPR the two sulfur donor atoms of the ene-1,2-dithiolatespectrum of (Tp*)MoO(bdtCl2) at room temperature chelate ligand (bdtCl2) are constrained to be mutuallydisplayed a single strong signal and six surrounding cis to each other by the fac stereochemistry imposed bysatellite lines that are characteristic of a mononuclear the tridentate Tp* ligand. This is the third such struc-Mo(V) center with the naturally abundant distribution turally characterized six-coordinate oxo-Mo(V) com-of isotopes and with Žg =1.973 and 95,97Mo ŽA = plex that contains a single ene-1,2-dithiolate ligand cis38.1 ×10 − 4 cm − 1 [18,21,27,28,32 – 35]. The isotropic to the terminal oxo group. The structures ofŽg value obtained from the room temperature solution (Tp*)MoO(bdt) [20,21] and (Tp*)MoO(qdt) [36] havespectrum agrees well with the mean of the anisotropic gi been previously reported.values, gi/3= 1.971, measured directly from the frozen The structural parameters of the {(Tp*)MoO}2 +solution EPR. Table 2 compares the EPR parameters core in (Tp*)MoO(bdtCl2) agree with those found forfor (Tp*)MoO(bdtCl2) to related (Tp*)MoO(S S) com- other oxo-Mo(V) complexes [18,20,21,29,32,33,36–41].plexes. The rhombic frozen glass EPR spectrum of , The observed Mo O distance (1.679(3) A) is similar to(Tp*)MoO(bdtCl2) exhibits parameters that are quite , that of (Tp*)MoO(bdt) (1.678(4) A) [20,21], (Tp*)Mo-similar to related (Tp*)MoO(S S) complexes [18,21,35]. , O(qdt) (1.6865(18) A) [36], and (Tp*)MoO(SPh)2However, (Tp*)MoO(qdt) (4), which contains a quinox- , (1.676(4) A) [18]. The terminal axial oxo ligand, asaline dithiolate rather than a substituted benzene dithi- expected, exerts a strong trans influence, lengtheningolate has a somewhat larger g3 value (Table 2) [35]. , , the Mo N31 distance (2.391(3) A) by 0.214 A relativeNonetheless, the EPR data of Table 2 further supportthe view that the EPR parameters of oxo-Mo centersare primarily determined by the inner coordinationenvironment of the molybdenum center [18]. For thesed1 (Tp*)MoO(S S) complexes containing monoene-1,2-dithiolate coordination, these EPR results indicate thatthe unpaired electron is localized in a HOMO that ispredominantly metal dxy in character, and whose or-bital composition is relatively unperturbed by remotesubstituents [18,19,28].3.2. X-ray structure analysis of(Tp*)MoO(bdtCl2) ·1 /2(C6H14) Fig. 3. The ORTEP drawing of (Tp*)MoO(bdtCl2) (3). The atoms are The structure of (Tp*)MoO(bdtCl2) (3), determined drawn as 50% probability ellipsoids. H-atoms have been made arbi-by single-crystal X-ray diffraction, is shown in Fig. 3. trarily small for clarity.
  • 6. F.E. Inscore et al. / Inorganica Chimica Acta 331 (2002) 246–256 251Table 3 The most significant difference between the structuresImportant bond lengths , (A) and bond angles (°) for of (Tp*)MoO(bdtCl2) (3) and (Tp*)MoO(bdt) (1) is the(Tp*)MoO(bdtCl2) (3) angle (l) formed between the ene-1,2-dithiolateBond lengths Bond angles (S C C S) least squares plane and the S Mo S plane along the line of intersection containing the S···S atomsMo(1) O(1) 1.679(3) O(1) Mo(1) N(11) 91.60(12) (see Fig. 4). This angle in 3 is 173.1°, and thus theMo(1) N(11) 2.166(3) O(1) Mo(1) N(21) 91.87(12) S C C S plane is folded up towards the terminal oxoMo(1) N(21) 2.177(3) N(11) Mo(1) N(21) 86.17(11)Mo(1) S(2) 2.3683(12) O(1) Mo(1) S(2) 100.99(10) group by q= 180° –l= 6.9°. The (bdtCl2) ring structureMo(1) S(1) 2.3805(12) N(11) Mo(1) S(2) 94.92(8) itself is essentially planar. The fold angle forMo(1) N(31) 2.391(3) N(21) Mo(1) S(2) 167.05(8) (Tp*)MoO(bdt) is considerably greater with q=S(2) C(2) 1.750(4) O(1) Mo(1) S(1) 101.81(10) 21.3(1)° [20]. This deformation in (Tp*)MoO(bdt) hasS(1) C(1) 1.753(4) N(11) Mo(1) S(1) 166.47(8) been previously attributed to a steric interaction be-C(2) C(1) 1.406(5) N(21) Mo(1) S(1) 91.54(8) S(2) Mo(1) S(1) 84.38(4) tween the C36 methyl group of the trans N31 pyrazolylS···S bite 3.190 O(1) Mo(1) N(31) 166.79(11) ring and the C1 and C2 atoms of the chelate ring [21]. distance The C36···C1 and C36···C2 non-bonded contact dis-Mo distance 0.258 N(11) Mo(1) N(31) 78.40(11) , tances in (Tp*)MoO(bdt) are 3.58 and 3.57 A, respec- from N2S2 tively, and significantly greater than the corresponding plane , distances of 3.375 and 3.345 A in (Tp*)MoO(bdtCl2). The estimated van der Waals contact between a methyl , group and an aromatic ring is 3.72 A [42]. Thus, non-bonded contact in (Tp*)MoO(bdtCl2) would seem- ingly favor a larger fold angle, as was observed in (Tp*)MoO(bdt). The packing of the molecules in the unit cell for each of these systems was also examined and it was determined for 3 that the C36 methyl groups from neighboring molecules were well separated from , the C1 and C2 atoms (greater than 8 A). The packing diagram showed that the closest Cl···Cl distance be- Fig. 4. Definition of the S S fold angle (q). , tween neighboring molecules was 3.920 A, greater than , the van der Waals contact of 3.60 A for chlorines [43]. , ,to Mo N21 (2.177(2) A) and 0.225 A relative to The results for the packing diagram analysis suggest ,Mo N11 (2.166(3) A). This effect is slightly greater that non-bonded intermolecular contacts are not thethan that observed in the (Tp*)MoO(bdt) complex cause of the observed fold angle deviation. The originwhere the Mo N31 bond was found to be 2.372(4) A , and contributions to this fold angle in (Tp*)MoO(bdt),[20]. The average Mo S distance (2.3744(12) A) in , (Tp*)MoO(bdtCl2), and (Tp*)MoO(qdt) are now being(Tp*)MoO(bdtCl2) is indistinguishable from that ob- investigated through DFT calculations [44]. Compari- ,served in (Tp*)MoO(bdt) (2.373(2) A). These Mo O son of the bond distances within the S C C S chelateand Mo S bond length comparisons for (Tp*)Mo- rings of 1 and 3 shows that the C1 C2 distance of 1 ofO(bdtCl2) and (Tp*)MoO(bdt), clearly show that these , 1.395(8) A and the distance in 3 of 1.406(5) A are ,structural parameters are not significantly perturbed by similar. The average S C distances of 1.760(6) in 1 andremote ligand substituent effects. The bond angles , 1.751(4) A in 3 are also not significantly different fromabout the six-coordinate Mo atom deviate significantly one another.from that of octahedral geometry, as illustrated by In summary, the crystallographic structures for threeO Mo S1 =101.81(10)° and O Mo S2 = 100.99(10)°. different (Tp*)MoO(S S) systems (1, 3, 4) show thatThe average O Mo S angle of 101.40° is slightly peripheral substituents on the ene-1,2-dithiolate chelategreater than that reported for (Tp*)MoO(bdt) (100.95°) induce no significant structural changes of the inner[20]. The molybdenum atom is displaced 0.258 A , coordination sphere of the molybdenum center. Theseabove the mean N2S2 equatorial plane in the direction results are consistent with other studies of Mo-ene-1,2-of the apical oxo ligand. This is slightly less than dithiolates obtained by X-ray and EXAFS that show ,observed for (Tp*)MoO(bdt) at 0.264 A [20]. The little differences in Mo S distance with changes of theS Mo S chelate bite angle of 84.38(4)° is tighter by substituents on the ene-1,2-dithiolate [45]. However, the0.74° relative to the corresponding angle (85.12(6)°) in substantial difference in the fold angle (q) of the ene-(Tp*)MoO(bdt) [20]. The S S non-bonded contact dis- dithiolate chelate ring between (Tp*)MoO(bdtCl2) and ,tance was calculated to be 3.190 A; this is only 0.021 A , (Tp*)MoO(bdt), warrants further investigation. Thesmaller than that observed for (Tp*)MoO(bdt) (3.211 structures of 1 and 3 suggest that this geometric feature ,A). may be related to subtle differences in the overall
  • 7. 252 F.E. Inscore et al. / Inorganica Chimica Acta 331 (2002) 246–256electronic structure of the (Tp*)MoO(S S) systems. It is tral features observed for 1 and 3 provides the basis forpossible that this fold angle may play an important the same band assignments being transferred to 3. Theregulatory role in the reactivity of pyranopterin Mo transition energies, intensities and band assignments ofenzymes during catalytic turnover. DFT calculations are the Gaussian resolved absorption spectra of 3 and 1 arein progress to determine the effects of this fold angle on presented in Table 4 for comparative purposes. Theelectronic structure in these well characterized most important spectral feature to recognize is the band(Tp*)MoO(S S) compounds [44]. centered at 19 000 cm − 1 in these (Tp*)MoO(S S) compounds, which has been assigned as the in-plane (ip)3.3. Electronic absorption spectroscopy pseudo-s (Sip “ Mo dxy ) CT transition [16].1 The inten- sity of this Sip “ Mo dxy transition probes the dominant The 298 K electronic absorption spectrum of 3 in covalency contributions between the Mo dxy redox ac-dichloroethane is shown in Fig. 5, and is very similar to tive orbital and the dithiolate Sip molecular orbitalsthose observed for other (Tp*)MoO(S S) complexes [16].2,3 The charge-transfer intensity of the Sip “Mo dxy[16–21,23]. The observed band patterns appear to be transition is observed to be very similar between 1 andcharacteristic of the (Tp*)MoO(S S) complexes re- 3, which indicates nearly equivalent pseudo-s mediatedported to date, which possess a five-membered chelate charge donation (covalency) between the in-plane ene-ring formed between the ene-1,2-dithiolate and molyb- 1,2-dithiolate donor orbitals of the bdt and bdtCl2denum atom [16,19]. Detailed electronic structure stud- ligands and the oxo-Mo dxy acceptor orbital. The similaries of (Tp*)MoO(bdt) (1) and (Tp*)MoO(tdt) (2) have energies of this transition reflect that the strength of thisresulted in the assignment of observed absorption bands bonding interaction, and thus the covalent destabiliza-below 26 000 cm − 1 as S“ Mo CT transitions [16,19]. tion of the Mo dxy redox orbital, remains essentially invariant and is little perturbed by remote substituentThe absorption features below 26 000 cm − 1 in 3 are effects on the benzene dithiolate ring. Another interest-nearly identical in energies and intensities relative to the ing aspect of the absorption spectra of 1 and 3 is thatobserved bands in 1. The close correspondence in spec- the energy and intensity for the Sop “ Mo dxz,yz CT transitions that occur at higher energy are not signifi- cantly different for the two compounds. The very similar absorption spectra observed among 1, 3 and 4 suggests these complexes possess nearly identical electronic struc- tures that are not significantly perturbed by remote substituents on the benzene ring. This conclusion is also consistent with the near identity of their EPR parame- ters (Table 2). However, the slight differences observed in the spectra of 4 with respect to 1 are perhaps reasonable because the qdt ligand possesses a benzene ring fused with a quinoxaline ring containing nitrogens [16,17,23]. The electron-donor properties of the ene-1,2- dithiolate have been discussed in detail with respect to the observed differences in the spectroscopic and electro- chemical data between 2 and 4 [23]. The origin of theseFig. 5. The 293 K electronic absorption spectrum of spectral differences and their relationship to reduction(Tp*)MoO(bdtCl2) (3) in 1,2-dichloroethane ( 10 − 4 M). potentials and ionization potentials remain an important area of research.Table 4Electronic absorption spectral data a 3.4. Solution reduction potentialsBand (Tp*)MoO(bdt) (1) b (Tp*)MoO(bdtCl2) (3)assignment The electrochemical properties of 3 have been exam- ined by cyclic voltammetry (CV) and are summarized in Energy m Energy m (cm−1) (M−1 cm−1) (cm−1) (M−1 cm−1)Sop “ Mo dxy 9100 360 9700 440 1 The pseudo-s interaction refers to the three-center bonding inter-Sop “ Mo dxy 13 100 270 13 200 330 action that occurs between the Mo dxy based orbital and the in-planeSip “ Mo dxy 19 400 sh, 1220 19 400 sh, 1080 Sps LCAO of the dithiolate chelate.Sop “ Mo dxz,yz 22 100 sh, 2380 22 500 sh, 1370 2 Sip and Sop refer to primarily sulfur containing molecular orbitalsSop “ Mo dxz,yz 25 100 5280 25 000 5530 of a dithiolate chelate oriented parallel and perpendicular to the plane of the dithiolate chelate, respectively. a 3 Gaussian resolved data, dichloroethane solutions, sh = shoulder. The intensity of a CT transition is proportional to the square of b Data taken from Ref. [16]. the overlap integral Ž€M
  • 8. €L2 as described by E. Solomon [46].
  • 9. F.E. Inscore et al. / Inorganica Chimica Acta 331 (2002) 246–256 253Table 5Summary of electrochemical data for (Tp*)MoO(S S) systems a,bComplex Mo(VI/V) Mo(V/IV) E1/2 (mV) DEp (mV) ipc/ipa E1/2 (mV) DEp (mV) ipc/ipa(1) (Tp*)MoO(bdt) 534 74 1.160 −642 74 0.9654(2) (Tp*)MoO(tdt) 462 76 1.051 −650 76 0.9516(3) (Tp*)MoO(bdtCl2) 742 64 2.589 −508 77 0.9969(4) (Tp*)MoO(qdt) c −422 a Conditions: cyclic voltammetry, 100 mV s−1, 0.5–1.0 mM sample, 0.1–0.2 M Bu4NBF4 in 1,2-dichloroethane. b Potentials vs. ferrocene/ferrocenium couple. c Data from Ref. [23].Table 5. The cyclic voltammograms of 3 and of related to the oxidation of the coordinated toluene-3,4-dithio-(Tp*)MoO(S S) compounds listed in Table 5 exhibit late ligand in a separate study of 2 [49]. The moreboth a quasi-reversible one-electron reduction wave and positive potential for the oxidation process of 3 relativea quasi-reversible one-electron oxidation wave in to 1 is consistent with their relative reduction poten-dichloroethane solution (25 °C) [47]. The chemical re- tials. This correlation suggests that 4 would be the mostversibility determined by peak current ratios are near difficult to oxidize and that the potential could be wellunity (ipa/ipc :1.0), but the anodic to cathodic peak outside of the accessible solvent window. A previouspotential separations (DEp in Table 5) are somewhat electrochemical study of 4 in 1,2-dichloroethane did notlarger than the ideal value of 59 mV expected for a exhibit a well-defined quasi-reversible oxidative wavecompletely reversible process. The quasi-reversibility of within the potential window for this solvent [23]. Stud-the reduction and oxidation couples indicates that min- ies are underway to investigate in greater detail theimal structural rearrangement occurs during the electro- electrochemical properties of the (Tp*)MoO(S S) com-chemical processes. plexes listed in Table 5 [47]. The well-defined reduction wave of 3 occurs at − 508 The electrochemical potentials in the (Tp*)MoVO-mV versus the Fc/Fc+ reference couple and can be (S S) complexes listed in Table 5 shift significantly asassigned to the Mo(V)/Mo(IV) redox couple that is a the nature of the ene-1,2-dithiolate (S S) ligandcharacteristic feature typical of the {(Tp*)MoVO}2 + changes. The electrochemical reduction or oxidation ofcomplexes possessing various equatorial donor ligands 3 and related (Tp*)MoVO(S S) compounds results for-[18]. The reduction potential of 3 is more positive than mally in the addition or removal of an electron fromthat for 1 and 2, but less positive than that for 4. The the HOMO, primarily Mo (dxy )1. The redox potentialoverall trend of Ered is 2 B1 B 3 B4. data of the (Tp*)MoVO(S S) series of complexes shows The cyclic voltammograms of 1 and 2 are unusual that the ease of reduction and difficulty in oxidationwith respect to the majority of {(Tp*)MoVO}2 + com- increases as the ene-1,2-dithiolate ligand becomes in-pounds investigated in that they also exhibit a well- creasingly more electron-withdrawing. The more posi-defined quasi-reversible oxidation wave at a potential of tive reduction potential for 3 relative to 1 suggests a+ 534 and +462 mV versus the Fc/Fc+ internal refer- decreased electron density on the Mo(V) center of 3 dueence couple, respectively. A similar oxidative wave (+ to the electron-withdrawing nature of the bdtCl2 ligand,732 mV) was observed for 3, but was ill-defined as it which decreases the electron density on the sulfur donoroccurred at the edge of the solvent window. The exact atoms coordinated to Mo, thereby stabilizing thenature of this oxidative wave remains to be determined. HOMO and making reduction easier as compared toOne reasonable possibility is the formal one-electron the unsubstituted bdt ligand. Likewise, the relativelyMo(V/VI) oxidation of the metal center. However, the greater density on the sulfur donor atoms in 1 destabi-oxidation could also be centered on the sulfur donor lize its HOMO energy relative to 3 and make oxidationatoms with no formal change in Mo oxidation state. A of 1 easier than 3. Similar remote ligand effects on thethird possibility is a coupled internal redox process in electron-donor properties of the ene-1,2-dithiolate lig-which the S centers are effectively oxidized by two and have been discussed in previous spectroscopic stud-electrons and the Mo reduced by one-electron to ies of 4 in order to understand the large positive ( 220Mo(IV). Such internal redox reactions have been ob- mV) shift in its reduction potential relative to 2 [23].served in other Mo S systems [48]. This oxidation wave However, it is not quite clear from the spectroscopicwas not observed in previous studies of 1 and 2 in data, why there is such a shift in reduction potentialacetonitrile solutions [18,20]. However, a reversible one- ( 150 mV) between 1 and 3. The similar absorptionelectron oxidation at 1.00 V (in CH3CN) was assigned intensity observed for the Sip “ Mo dxy CT transition in
  • 10. 254 F.E. Inscore et al. / Inorganica Chimica Acta 331 (2002) 246–256these two (Tp*)MoO(S S) complexes indicates that the and 3. Detailed studies employing DFT calculations arenature of this in-plane bonding interaction is essentially underway to further probe the electronic structure con-equivalent, and therefore cannot be the origin of the tributions to the reduction potentials and the nature ofsignificant reduction potential differences observed be- the donor/acceptor orbitals in 3 and relatedtween these complexes. Unlike the absorption spectrum (Tp*)MoO(S S) complexes [44].of 4, the intensities of the Sop “Mo dxz,yz CT transi-tions in 1 and 3 are nearly the same, indicating no 3.5. Gas-phase photoelectron spectroscopysignificant change in the nature of Mo-dithiolate bond-ing between 1 and 3. Thus, it appears that electronic The gas-phase He I valence photoelectron spectrastructure contributions to the valence ionization energy (5.7–15.7 eV) of 1 and 3 are presented in Fig. 6. The(VIE) of the ground-state HOMO (covalent destabiliza- ionizations exhibited in the region above 8.2 eV aretion of the redox orbital and reduction of the Mo characteristic of the {(Tp*)MoVO}2 + core observed ineffective nuclear charge via electron-donating ability of the PES of a variety of complexes possessing equatorialthe dithiolate) does not play a dominant role in the oxygen, halide and sulfur donor ligands studied previ-observed reduction potential differences between 1 and ously [22,23,50,51]. The ionizations in this spectral re-3. Solvation effects and changes in the reorganization gion for 1 and 3 are associated primarily with removalenergy of the complex and the solvent can also con- of electrons from the orbitals of the (Tp*) ligand, thetribute to the potential differences observed between 1 benzene ring, and lone pairs of the oxo and chloride atoms. The large number and highly overlapping nature of these ionizations make unambiguous assignment of the spectral features in this region difficult. However, the spectral differences between 1 and 3 in this region are mainly due to the chloride lone pair ionizations at 11 eV in 3, which are absent in the spectrum of 1. The greatly reduced intensity of this particular ioniza- tion in the He II spectrum of 3 supports this assignment [44]. The spectra of 1 and 3 show two ionizations below 8.2 eV that are energetically isolated from the ligand- based bands. The two spectral features in this region are similar to those observed in 2 and 4 [23]. The ionization energies for these two bands observed in 3 and related (Tp*)MoO(S S) compounds are presented in Table 6. The energies of the first ionization band for the (Tp*)MoO(S S) series increase in the order, 2B 1B 3B 4. The first band has been attributed to the ionization of the Mo (dxy )1 electron from these formally Mo(V) compounds [22,23]. Thus, the valence ionization energy of the Mo (dxy ) orbital is of particular interest as it is the active orbital associated with the metal-cen-Fig. 6. He I photoelectron spectroscopy of (Tp*)MoO(bdtCl2) (3) tered redox processes occurring in these oxomolybde-(top) and (Tp*)MoO(bdt) (1) (bottom). The first ionization, centered num monoene-1,2-dithiolate complexes. The ionizationaround 7.2 eV, is associated with metal and sulfur based molecular energies show that the first band in 3 is stabilized byorbitals. Complex 3 has ionizations at 11 eV assigned to chlorinebased orbitals. Both complexes show pyrazolyl ring based ionizations 0.29 and 0.16 eV relative to 2 and 1, respectively. Thecentered around 9 eV, as previously assigned [50,51]. position of the first band in 3 has been found to be destabilized by 0.48 eV from the first ionization energyTable 6 of 4. These results clearly demonstrate the effects ofComparison of He I gas-phase spectral data for (Tp*)MoO(S S) remote ligand substituents on the ionization energies ofsystems the metal-based HOMO. The shifts in these gas-phaseComplex Ionization energy (eV, 9 0.2 eV) ionization energies follow trends in redox potentials measured in solution. Thus, the increasing electron Band A Band B Reference withdrawing properties of the dithiolate ligand and ease in reduction of 3 relative to 1, are consistent with the(1) (Tp*)MoO(bdt) 7.08 7.57 This work, [44] increase in ionization energy (stabilization) of the(2) (Tp*)MoO(tdt) 6.95 7.5 [22,23](3) (Tp*)MoO(bdtCl2) 7.24 7.66 This work, [44] HOMO in 3 compared to that observed in 1. Fig. 7(4) (Tp*)MoO(qdt) 7.72 8.04 [23] presents the equation for the least-squares line of com- pounds 1, 2, and 3, where IE (first ionization energy,
  • 11. F.E. Inscore et al. / Inorganica Chimica Acta 331 (2002) 246–256 255 olate. Solvation effects, reorganizational energy changes, and covalent reduction of the effective nuclear charge of the Mo ion due to charge transfer differences involving higher energy acceptor orbitals are currently being investigated as possible contributions to redox potential differences. The crystal structures now avail- able for compounds 1, 3, and 4 provide a framework for high-level DFT calculations to be initiated and evaluated within the context of existing spectroscopic and electrochemical data [44]. Such calculations should provide detailed insight into the geometric and elec- tronic structures of these oxo-Mo monoene-1,2-dithio- late centers and help to define structure/function correlations for the active sites of the pyranopterin MoFig. 7. Correlation of half-wave oxidation potential with lowest enzymes.energy ionization potential for the (Tp*)MoO(S S) compounds ofTable 6. 4. Supplementary dataeV) = 0.976(Eox, V) +6.52 with a correlation coefficientof 0.977. These data clearly show a correlation between Crystallographic data for the structural analysis havethe first gas-phase ionization energy and the solution been deposited with the Cambridge Crystallographicoxidation potential, and indicates that the nature of Data Centre, CCDC No. 174598. Copies of this infor-these processes is similar. The second ionization band mation may be obtained free of charge from Thein the (Tp*)MoO(S S) complexes has no counterpart in Director, CCDC, 12 Union Road, Cambridge, CB2the spectra of analogous alkoxide or diolato com- 1EZ, UK (fax: +44-1223-336-033; e-mail: deposit@pounds [50,51] and has been tentatively assigned to the ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).removal of electrons from dithiolate-based orbitals lo-calized on the sulfur donor atoms of the ene-1,2-dithio-late chelate. The PES data presented in Table 6 here for Acknowledgements3 and other (Tp*)MoO(S S) complexes listed promiseconsiderable insight into Mo S covalency and the pos- The X-ray diffraction studies were carried out usingtulated role of the ene-1,2-dithiolate as an electronic the facilities of the Molecular Structure Laboratory,buffer [22]. However, additional data are required from Department of Chemistry, University of Arizona undermulti-wavelength sources and high-level DFT calcula- the direction of Dr. Michael Carducci; mass spectrations in order to analyze in detail the observed spectral were recorded at the University of Arizona Mass Spec-features and this work is currently in progress [44]. trometry Facility; EPR studies were carried out at the University of Arizona under the direction of Dr.3.6. Summary Arnold Raitsimring. PES studies were carried out at the Center for Gas-Phase Electron Spectroscopy, Univer- The synthesis and characterization of the new com- sity of Arizona under the direction of Dr. Nadineplex, (Tp*)MoO(bdtCl2) (3) have provided an addi- Gruhn. We thank Professor Martin Kirk and Nicktional oxo-Mo monoene-1,2-dithiolate system for direct Rubie at the University of New Mexico for helpfulcomparison with the structurally characterized com- discussions and quantitative absorption spectral data.pounds (Tp*)MoO(bdt) (1) and (Tp*)MoO(qdt) (4). We gratefully acknowledge support by the NationalThe most significant structural difference for 3 is the Institutes of Health (Grant GM-37773).much smaller fold angle (q) between the MoS2 andS C C S planes. Variations in q may modulate the Referenceselectronic structure of these systems and could play animportant regulatory role in the pyranopterin Mo en- [1] R. Hille, Chem. Rev. 96 (1996) 2757.zymes during catalysis. The solution redox potentials [2] C. Kisker, H. Schindelin, A. Pacheco, W.A. Wehbi, R.M. Gar-and the gas-phase ionization energies clearly demon- rett, K.V. Rajagopalan, J.H. Enemark, D.C. Rees, Cell 91 (1997)strate the sensitivity of the (Tp*)MoO(S S) system to 973.remote ligand effects. However, IR, EPR, and elec- [3] M.J. Romao, M. Archer, I. Moura, J.J.G. Moura, J. LeGall, R. ˜ Engh, M. Schneider, P. Hof, R. Huber, Science 270 (1995) 1170.tronic absorption spectroscopies suggest that the elec- [4] R. Huber, P. Hof, R.O. Duarte, J.J.G. Moura, I. Moura, M.Y.tronic structure of 1 and 3 remains relatively Liu, J. LeGall, R. Hille, M. Archer, M.J. Romao, Proc. Natl. ˜unperturbed by peripheral ligation to the ene-1,2-dithi- Acad. Sci. USA 93 (1996) 8846.
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