Spectroscopic Evidence for a Unique Bonding Interaction in Oxo-Molybdenum Dithiolate Complexes: Implications for σ Electron Transfer Pathways in the Pyranopterin Dithiolate Centers of Enzymes

Spectroscopic Evidence for a Unique Bonding Interaction in Oxo-Molybdenum Dithiolate Complexes: Implications for σ Electron Transfer Pathways in the Pyranopterin Dithiolate Centers of Enzymes

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  Spectroscopic Evidence for a Unique Bonding Interaction in Oxo-Molybdenum DithiolateComplexes: Implications for  σ   Electron Transfer Pathways in the Pyranopterin DithiolateCenters of Enzymes Frank E. Inscore, † Rebecca McNaughton, † Barry L. Westcott, ‡ Matthew E. Helton, † Robert Jones, † Ish K. Dhawan, ‡ John H. Enemark,* ,‡ and Martin L. Kirk* ,† Department of Chemistry, The University of New Mexico, Albuquerque, New Mexico 81731-1096, andDepartment of Chemistry, The University of Arizona, Tucson, Arizona 85721-0041  Recei V  ed September 18, 1998  Solution and solid state electronic absorption, magnetic circular dichroism, and resonance Raman spectroscopieshave been used to probe in detail the excited state electronic structure of LMoO(bdt) and LMoO(tdt) (L ) hydrotris-(3,5-dimethyl-1-pyrazolyl)borate; bdt ) 1,2-benzenedithiolate; tdt ) 3,4-toluenedithiolate). The observed energies,intensities, and MCD band patterns are found to be characteristic of LMoO(S-S) compounds, where (S-S) is adithiolate ligand which forms a five-membered chelate ring with Mo. Ab initio calculations on the 1,2-ene-dithiolate ligand fragment,  - SC d CS - , show that the low-energy S f  Mo charge transfer transitions result fromone-electron promotions srcinating from an isolated set of four filled dithiolate orbitals that are primarily sulfurin character. Resonance Raman excitation profiles have allowed for the definitive assignment of the ene-dithiolateS in - plane f  Mo d  xy  charge transfer transition. This is a bonding-to-antibonding transition, and its intensity directlyprobes sulfur covalency contributions to the redox orbital (Mo d  xy ). Raman spectroscopy has identified threetotally symmetric vibrational modes at 362 cm - 1 (S - Mo - S bend), 393 cm - 1 (S - Mo - S stretch), and 932 cm - 1 (Mo t O stretch), in contrast to the large number low-frequency modes observed in the resonance Raman spectrumof   Rhodobacter sphaeroides  DMSO reductase. These results on LMoO(S-S) complexes are interpreted in thecontext of the mechanism of sulfite oxidase, the modulation of reduction potentials by a coordinated ene-dithiolate(dithiolene), and the orbital pathway for electron transfer regeneration of pyranopterin dithiolate Mo enzymeactive sites. Introduction The pterin-containing molybdenum enzymes catalyze avariety of two-electron redox reactions coupled to formal oxygenatom transfer 1 - 10 but, unlike monooxygenases, the oxygen atominvolved in catalysis derives from water instead of dioxygen.X-ray crystallography has begun to define the salient structuralfeatures of these enzyme active sites, with the structures of xanthine oxidase related aldehyde oxidoreductase from  Desul- fo V  ibrio gigas , 11,12 the dimethyl sulfoxide (DMSO) reductasesfrom  Rhodobacter capsulatus 13,14 and  Rhodobacter sphaeroi-des , 15 and chicken liver sulfite oxidase 16 all having been recentlydetermined. These studies have revealed that the active sitesgenerally possess at least one Mo t O unit in the oxidized Mo-(VI) state, and all appear to contain at least a single pyranop-terin 17 (Figure 1) which is coordinated to Mo via an ene-dithiolate (dithiolene) linkage. Although controversy still existswith respect to whether the crystallographically determinedstructures reveal the catalytically competent enzyme active site † The University of New Mexico. ‡ The University of Arizona.(1) Hille, R.  Chem. Re V  .  1996 ,  96  , 2757 - 2816.(2) Young, C. G.; Wedd, A. G.  J. Chem. Soc., Chem. Commun.  1997 ,1251 - 1297.(3) Stiefel, E. I.  J. Chem. Soc., Dalton Trans.  1997 , 3915 - 3923.(4) Enemark, J. H.; Young, C. G.  Ad  V  . Inorg. Chem.  1993 ,  40 , 1 - 88.(5) Pilato, R. S.; Stiefel, E. I. In  Bioinorganic Catalysis ; Reedijik, J., Ed.;Dekker: New York, 1993; pp 131 - 188.(6) Holm, R. H.  Coord. Chem. Re V  .  1990 ,  100 , 183 - 221.(7) Rajagopalan, K. V.  Ad  V  . Enzym. Relat. Areas Mol. Biol.  1991 ,  64 ,215 - 290.(8) Bastian, N. R.; Kay, C. J.; Barber, M. J.; Rajagopalan, K. V.  J. Biol.Chem.  1991 ,  266  , 45 - 51.(9) Johnson, J. L.; Bastian, N. R.; Rajagopalan, K. V.  Proc. Natl. Acad.Sci. U.S.A.  1990 ,  87  , 3190 - 3194.(10) Boyington, V. C.; Gladyshev, V. N.; Khangulov, S. V.; Stadtman, T.C.; Sun, P. D.  Science  1997 ,  275 , 1305 - 1308.(11) Roma˜o, M. J.; Archer, M.; Moura, I.; Moura, J. J. G.; LeGall, J.; Engh,R.; Schneider, M.; Hof, P.; Huber, R.  Science  1995 ,  270 , 1170 - 1176.(12) Huber, R.; Hof, P.; Duarte, R. O.; Moura, J. J. G.; Moura, I.; Liu, M.;LeGall, J.; Hille, R.; Archer, M.; Roma˜o, M. J.  Proc. Natl. Acad. Sci.USA  1996 ,  93 , 8846 - 8851.(13) Schneider, F.; Lo¨we, J.; Huber, R.; Schindelin, H.; Kisker, C.;Kna¨blein, J.  J. Mol. Biol.  1996 ,  263 , 53 - 69.(14) McAlpine, A. S.; McEwan, A. G.; Shaw, A. L.; Bailey, S.  J. Biol. Inorg. Chem.  1997 ,  2 , 690 - 701.(15) Schindelin, H.; Kisker, C.; Hilton, J.; Rajagopalan, K. V.; Rees, D.C.  Science  1996 ,  272 , 1615 - 1621.(16) Kisker, C.; Schindelin, H.; Pacheco, A.; Wehbi, W. A.; Garrett, R.M.; Rajagopalan, K. V.; Enemark, J. H.; Rees, D. C.  Cell  1997 ,  91 ,973 - 983.(17) The generic term “pyranopterin” has been proposed by R. Hille (  JBIC, J. Biol. Inorg. Chem.  1997 ,  2 , 804 - 809) for the tricyclic heterocycleof Figure 1. For a more detailed discussion of the nomenclature of this tricyclic system and its derivatives, see: Fischer, B.; Enemark, J.H.; Basu, P.  J. Inorg. Biochem.  1998 ,  72 , 13 - 21. Figure 1.  Structure of the pyranopterin derived from protein crystal-lographic studies (shown in protonated form). 10 - 16 1401  Inorg. Chem.  1999,  38,  1401 - 1410 10.1021/ic981126o CCC: $18.00 © 1999 American Chemical SocietyPublished on Web 03/19/1999  coordination geometry, the X-ray studies have provided pro-found insight toward the development of   consensus  structureswhich correlate the crystallographic results with availableEXAFS 18 - 21 and spectroscopic data. This has allowed forcategorizing these Mo enzymes into three main families on thebasis of the geometric structure of the oxidized active sites(Figure 2). 1,7 The crystallographic studies have concluded that the pyran-opterin adopts a distinctly nonplanar conformation with anapproximately 40 °  twist of the pyran ring, 10 - 16,22 stronglyfavoring a reduced pyranopterin which lacks extensive  π  -de-localization over the tricyclic ring. Although the pyranopterinis well known to anchor the pyranopterin to the protein viaextensive hydrogen bonding, 10 - 16,22 its precise functional rolein catalysis remains undetermined. The intimate association of the pyranopterin dithiolate with Mo suggests a variety of rolesfor the pyranopterin in catalysis, including modulating thereduction potential and acting as an electron transfer pathwayto other endogenous or exogenous redox partners. 1,7 There isstructural and reactivity evidence for the direct involvement of the pyranopterin in electron transfer. The structure of   D. gigas aldehyde oxidoreductase has the amino group of the pterinhydrogen bonded to a cysteinyl sulfur of a 2Fe2S cluster. 12 Also,sulfite oxidase is inactivated by ferricyanide oxidation of thepyranopterin to the fully oxidized state. 23 This study conclusivelyshowed that the presence of a fully oxidized pterin compromisedthe ability of the enzyme to reduce the physiological oxidant(cytochrome  c ) but did not significantly affect the ability of the enzyme to oxidize sulfite. Therefore, it appears that theoxidized pyranopterin severely retards the egress of reducingequivalents to the endogenous  b -type heme.Magnetic circular dichroism (MCD) spectroscopy has beenutilized in the study of desulfo-inhibited xanthine oxidase, 24 DMSO reductase, 25 and glycerol-inhibited DMSO reductase, 26 and the MCD spectra of the DMSO reductases have beeninterpreted in the context of out-of-plane S p π   f   Mo chargetransfer transitions. Additionally, resonance Raman spectros-copy 27 - 29 has been used to probe the lowest energy absorptionfeatures observed in the Mo(VI) and Mo(IV) oxidation statesof the  R. sphaeroides  enzyme and the vibrational spectra displaya complicated series of vibrational bands in the low-frequencyregion between 250 - 450 cm - 1 where metal - ligand vibrationsare expected to occur. These vibrational bands were shown tobe sensitive to  34 S isotopic substitution, indicating that theobserved modes possess a significant Mo - S stretching contri-bution, indicative of the S f  Mo charge transfer nature of thelow-energy absorption features. Curiously, the multitude of low-frequency vibrations found in DMSO reductase are absent inthe resonance Raman spectra of human sulfite oxidase and itsC207S mutant. 30 Regardless, the observation of S f  Mo chargetransfer transitions associated with the Mo-ene-dithiolate portionof the active site has provided the impetus for synthetic andelectronic structure studies of model compounds which mimicgeometric, spectroscopic, and electron transfer aspects of theactive site. Most notable among the spectroscopic work has beena seminal MCD and electronic absorption study which deter-mined that S f  Mo LMCT transitions dominate the low-energyabsorption and MCD spectra of oxomolybdenum ene-1,2-dithiolate model complexes. 31 The results of this study suggestedthat the oxomolybdenum ene-1,2-dithiolate moiety is a uniquelydesigned catalytic unit, allowing for Mo t O bond destabilizationand concomitant activation in oxygen atom transfer catalysisas well as providing an electron transfer conduit coupling Mowith the pyranopterin.X-ray crystallographic and XAS studies have defined thesalient structural features for the pyranopterin centers of Moenzymes. This allows for detailed spectroscopic studies to beinitiated on relevant model complexes in order to develop insightinto the electronic srcin of observed spectroscopic features andtheir contributions to reactivity. We have employed a combina-tion of electronic absorption, MCD, and resonance Ramanspectroscopies to understand the electronic structure of firstgeneration mono-oxo Mo(V) ene-1,2-dithiolate models forenzymes in the sulfite oxidase family. The results of thisspectroscopic study are significant with respect to understandingthe mechanism and determining the stereochemical location of the catalytically labile oxo group in sulfite oxidase; providingan emerging picture of how Mo redox potentials may bemodulated by a coordinated dithiolate; and describing the orbitalpathway for electron transfer regeneration of these Mo enzymeactive sites. (18) Cramer, S. P.; Gray, H. B.; Rajagopalan, K. V.  J. Am. Chem. Soc. 1979 ,  101 , 2772 - 2774.(19) George, G. N.; Kipke, C. A.; Prince, R. C.; Sunde, R. A.; Enemark,J. H.; Cramer, S. P.  Biochemistry  1989 ,  28  , 5075 - 5080.(20) Cramer, S. P.; Wahl, R.; Rajagopalan, K. V.  J. Am. Chem. Soc.  1981 , 103 , 7721 - 7727.(21) George, G. N.; Garrett, R. M.; Prince, R. C.; Rajagopalan, K. V.  J. Am. Chem. Soc.  1996 ,  118  , 8588 - 8592.(22) Chan, M. K.; Mukund, S.; Kletzin, A.; Adams, M. W. W.; Rees, D.C.  Science  1995 ,  267  , 1463 - 1469.(23) Gardlik, S.; Rajagopalan, K. V.  J. Biol. Chem.  1991 ,  266  , 4889 - 4895.(24) Peterson, J.; Godfrey, C.; Thomson, A. J.; George, G. N.; Bray, R. C.  Biochem. J.  1986 ,  233 , 107 - 110.(25) Benson, N.; Farrar, J. A.; McEwan, A. G.; Thompson, A. J.  FEBS  Lett.  1992 ,  307  , 169 - 172.(26) Finnegan, M. G.; Hilton, J.; Rajagopalan, K. V.; Johnson, M. K.  Inorg.Chem.  1993 ,  32 , 2616 - 2617.(27) Gruber, S.; Kilpatrick, L.; Bastian, N. R.; Rajagopalan, K. V.; Spiro,T. G.  J. Am. Chem. Soc.  1990 ,  112 , 8179 - 8180.(28) Kilpatrick, L.; Rajagopalan, K. V.; Hilton, J.; Bastian, N. R.; Steifel,E. I.; Pilato, R. S.; Spiro, T. G.  Biochemistry  1995 ,  34 , 3032 - 3039.(29) Garton, S. D.; Hilton, J.; Oku, H.; Crouse, B. R.; Rajagopalan, K. V.;Johnson, M. K.  J. Am. Chem. Soc.  1997 ,  119 , 12906 - 12916.(30) Garton, S. D.; Garrett, R. M.; Rajagopalan, K. V.; Johnson, M. K.  J. Am. Chem. Soc.  1997 ,  119 , 2590 - 2591.(31) Carducci, M. D.; Brown, C.; Solomon, E. I.; Enemark, J. H.  J. Am.Chem. Soc.  1994 ,  116  , 11856 - 11868. Figure 2.  Consensus active site structures for the oxidized Mo(VI)state of the three families of pyranopterin Mo enzymes according toHille 1 ; DMSO reductase family; xanthine oxidase family; sulfite oxidasefamily. 1402  Inorganic Chemistry, Vol. 38, No. 7, 1999  Inscore et al.  Experimental Section General.  Unless otherwise noted, all reactions were carried out inan inert atmosphere of nitrogen using Schlenk techniques. All solventswere dried by distillation, and deoxygenated prior to use. Purificationof solvents was accomplished using the following methodologies:pyridine and triethylamine from potassium hydroxide; toluene fromsodium benzophenone. Other solvents were used without furtherpurification. The compounds LMoOCl 2 , 32 LMo V O(tdt), 32 LMo V O-(bdt), 33,34 and LMo V O(edt) 32 were prepared as previously described. Abbreviations.  L, hydrotris(3,5-dimethyl-1-pyrazolyl)borate; bdt,1,2-benzenedithiolate; tdt, 3,4-toluenedithiolate; edt, 1,2-ethanedithio-late; H 2 qdt, quinoxaline-2,3-dithiol; qdt, 2,3-dithioquinoxaline). Preparation of LMoO(qdt).  The reagents 2,3-dihydroxyquinoxalineand phosphorus pentasulfide were purchased from Aldrich ChemicalCo. Quinoxaline-2,3-dithiol was prepared by a modified version of Morrison. 35,36 To a dry toluene solution of LMoOCl 2  (0.5 g, 1.05 mmol)was added a toluene solution containing 0.41 g (2.1 mmol) of H 2 qdtand 150  µ L (2.1 mmol) of triethylamine, dropwise by cannula at 70 ° C. This solution was allowed to react for approximately 5 h, and duringthis time the color of the solution changed from lime green to darkred. The resulting dark red solution was filtered and concentrated atreduced pressure to give a dark red powder, which was subsequentlyredissolved in a minimum amount of toluene and chromatographed onsilica gel. The compound eluted in a binary mixture of toluene/1,2-dichloroethane (9:1) as a red band. Yield  )  15%. Anal. Calcd forC 23 H 26 N 8 OS 2 BMo: C, 45.93; H, 4.36. Found: C, 45.04; H, 4.33. IR(KBr, cm - 1 ):  ν (Mo d O) 940,  ν (B - H) 2551. MS (FAB):  m  /   z  602.1(parent ion), 507 (parent  -  3,5-dimethylpyrazole), 410 (parent  - dithiolate). Physical Characterization.  Elemental analysis was performed atThe University of New Mexico using a Perkin-Elmer 2400 CHNelemental analyzer equipped with a P-E AD-6 Autobalance. MassSpectra were collected at The Nebraska Center for Mass Spectrometryin the Department of Chemistry at the University of Nebraska s Lincoln. Electronic Absorption Spectroscopy.  Mull and solution electronicabsorption spectra were collected on a double beam Hitachi U-3501UV-vis-NIR spectrophotometer capable of scanning a wavelengthregion between 185 and 3200 nm. All absorption spectra were collectedat 2.0 nm resolution in a single-beam configuration. The instrumentwas calibrated with reference to the 656.10 nm deuterium line.Immediately following acquisition of the sample spectra, backgroundspectra were collected to correct for residual absorption due to thesolvent or mulling agent and to correct for light scattering effects.Solution samples were prepared by dissolving the compounds indegassed dichloroethane. The electronic absorption spectra weresubsequently collected in 1 cm pathlength Helma quartz cells (black-masked Suprasil II, equipped with a Teflon stopper). Mull sampleswere prepared by grinding the solid sample into a fine powder beforedispersing it into poly(dimethylsiloxane). The prepared mull wassubsequently placed between two 1 mm thick Infrasil quartz discs(ESCO) and secured in a custom designed sample holder. A JanisSTVP-100 continuous flow cryostat mounted in a custom designedcradle assembly was used for acquisition of the low-temperature ( ∼ 5K) spectra. The sample temperature was continuously monitored witha Lakeshore silicon-diode (PT-470) and regulated by a combination of helium flow and dual heater assemblies. Gaussian resolution of spectralbands and corrections for light scattering were accomplished withKaleidaGraph and programs incorporated within the Hitachi versionof the Grams software package. Magnetic Circular Dichroism Spectroscopy.  Low-temperatureMCD data were collected on a system consisting of a Jasco J600 CDspectropolarimeter employing Hamamatsu photomultiplier tubes of either S-1 or S-20 response, an Oxford Instruments SM4000-7Tsuperconducting magneto-optical cryostat (0 - 7 Tesla and 1.4 - 300 K),and an Oxford Instruments ITC503 temperature controller. Thespectrometer was calibrated for CD intensity and wavelength usingcamphorsulfonic acid and a Nd-doped reference glass sample (SchottGlass). Solid-state MCD spectra were obtained by dispersing finelyground samples in poly(dimethylsiloxane) and compressing the suspen-sion between two 1 mm thick Infrasil quartz discs (ESCO). Depolar-ization of the incident radiation was checked by comparing thedifference in CD intensity of a standard Ni ( + )-tartrate solutionpositioned before and then after the sample. Samples which depolarizedthe light by < 5% were deemed suitable. The MCD spectra in the 250 - 800 nm range were obtained at 2.0 nm resolution, and data between400 and 1050 nm were collected at a fixed slit width of 150  µ . AllMCD spectra were collected in an applied magnetic field of 7 Tesla. Vibrational and Resonance Raman Spectroscopy.  Infrared spectrawere recorded on a BOMEM MB-100 FT-IR spectrometer as pressedKBr disks. The infrared spectra were utilized to monitor the purity of the compounds, as indicated by the absence of the 962 cm - 1 Mo t Ostretch associated with the LMoOCl 2  precursor complex. 32 Resonance Raman spectra were collected in a 135 °  backscatteringgeometry. A Coherent Innova 70-5 (5W) Ar + ion laser was the photonsource (457.9 - 528.7 nm, 9 discrete lines) for inducing Ramanscattering. The scattered radiation was dispersed onto a liquid N 2  cooled1 ′′  Spex Spectrum One CCD detector using a Spex 1877E triple gratingmonochromator equipped with 600, 1200, and 1800 gr/mm holographicgratings at the spectrographic stage. The laser power at the sample waskept between 40 and 100 mW in order to prevent possible photo- andthermal degradation of the sample. Solid samples were prepared asfinely ground powders and dispersed in a NaCl(s) matrix with Na 2 SO 4 added as an internal standard. These samples were subsequently sealedin an NMR tube and Raman spectra were obtained by spinning thesample in a modified NMR sample holder/spinner. The samples weremaintained at ∼ 140 ( 10 K by the use of a custom designed cold N 2 gas flow system. The sample temperature was periodically monitoredwith a Lakeshore silicon diode (PT-470) enclosed in a separate NMRtube. The construction of resonance Raman profiles was accomplishedby comparing the integrated intensity of a Raman band at a givenexcitation wavelength relative to that of the 992.4 cm - 1 band of Na 2 -SO 4 . All data were scan averaged, and any individual data set withvibrational bands compromised by cosmic events was discarded.Solution Raman spectra were obtained in degassed benzene and spunin a sealed NMR tube at room temperature. Depolarization ratios wereobtained by placing a rotatable polarizer before the polarizationscrambler and monochromator entrance slit. Relative Raman intensities(perpendicular and parallel to incident radiation) for a given Ramanband were measured relative to the 992 cm - 1 band of benzene. Ab Initio Calculations.  Ab initio calculations were performed usingthe Gaussian 94 suite of programs. 37 A 6-31G** basis set was employedin calculating the energies and wavefunctions for the model 1,2-ene-dithiolate ( - SCH d CHS - ). ResultsSolution Electronic Absorption Spectra.  Figure 3 depictsthe room temperature electronic absorption spectrum of LMoO-(bdt) between 6000 and 35 000 cm - 1 in dichloroethane. Thespectrum is very similar to that previously reported for LMoO-(tdt). 31 However, the transitions observed for LMoO(tdt) aregenerally shifted to slightly lower energies relative to thecorresponding bands in LMoO(bdt). The low-energy region of the spectrum consists of three distinct spectral features (bands 1 ,  2 , and  4 ) below ∼ 20 000 cm - 1 . We have found these bandsto be characteristic of LMoO(S-S) compounds, where S-S is adithiolene or dithiolate ligand which forms a five-memberedchelate ring with Mo. Band  3  is very weak, and only discerniblein the low-temperature MCD spectra ( V  ide infra ). The transitionenergies and molar extinction coefficients for four LMoO(S-S) (32) Cleland, W. E., Jr.; Barnhart, K. M.; Yamanouchi, K.; Collison, D.;Mabbs, F. E.; Ortega, R. B.; Enemark, J. H.  Inorg. Chem.  1987 ,  26  ,1017 - 1025.(33) Dhawan, I. K.; Enemark, J. H.  Inorg. Chem.  1996 ,  35 , 4873 - 4882.(34) Dhawan, I. K.; Pacheco, A.; Enemark, J. H.  J. Am. Chem. Soc.  1994 , 116  , 7911 - 7912.(35) Morrison, D. C.; Furst, A.  J. Org. Chem.  1956 ,  21 , 470 - 471.(36) Helton, M. E.; Kirk, M. L. Submitted for publication. (37) Gaussian Incorporated, Pittsburgh, PA. Unique Bonding in Oxo-Molybdenum Dithiolates  Inorganic Chemistry, Vol. 38, No. 7, 1999  1403  complexes are presented in Table 1 for comparative purposes.Of particular interest is band  4 , which is the first absorptionfeature possessing appreciable intensity characteristic of a chargetransfer transition. Solid-State Electronic Absorption and MCD Spectra.  The5 K mull MCD/absorption overlay of LMoO(bdt) is shown inFigure 4. The electronic absorption spectrum exhibits fivedistinct features in the solid state with weak to significantintensity. The 21 500 cm - 1 band is observed as a reproducibleshoulder in the mull absorption of LMoO(bdt) but is conspicu-ously absent in the corresponding absorption spectrum of LMoO(tdt). The general similarity of the solution and mullabsorption spectra for LMoO(bdt) indicate that only minorstructural changes accompany solvation. This is true for all of the LMoO(S-S) compounds listed in Table 1.The MCD spectrum of LMoO(bdt) is composed of bothC-terms and pseudo A-terms. 38 MCD C-terms possess absorptivebandshapes with intensity maxima at the same energy ascorresponding absorption features, while pseudo A-terms possessderivative-shaped dispersions which possess zero intensity atan energy corresponding to an absorption maximum. Therelationship between observed MCD and electronic absorptionbands in LMoO(bdt) is given in Table 2.Figure 5 compares the 5 K/7 T MCD mull spectra of LMoO-(bdt) and LMoO(tdt) in the spectral region between 10 000 and40 000 cm - 1 . The MCD spectra are seen to be quite similar,and this band pattern is characteristic of LMoO(S-S) complexeswhere the Mo t O bond is oriented cis to a single dithiolateligand which forms a five-membered chelate ring with Mo. 31 However, noticeable differences in MCD sign exist for thesecompounds in the 14 000 - 17 000 cm - 1 range, where nodiscernible maxima occur in the absorption spectra (see Figure3). Close inspection of Figure 5 reveals the reason the 21 500cm - 1 transition in LMoO(bdt) is not resolved in the MCD orabsorption spectra of LMoO(tdt). The positive C-term at 19 300cm - 1 and the low-energy negatively signed component of the24 300 cm - 1 positive pseudo A-term observed in LMoO(bdt)are energetically compressed in the MCD spectrum of LMoO-(tdt), effectively masking the 21 500 cm - 1 spectral feature. TheMCD spectrum of LMoO(qdt) shows this transition as a clearlyresolved positive pseudo A-term. 36 Vibrational Spectra.  The IR data for LMoO(bdt) and LMoO-(tdt) display intense peaks at 932 and 926 cm - 1 , respectively.Strong IR bands in the 910 - 965 cm - 1 range have been reportedfor a variety of LMoOX 2  complexes, 39 - 43 and this band has (38) Piepho, S. B.; Schatz, P. N.  Group Theory in Spectroscopy with Applications to Magnetic Circular Dichroism ; Wiley-Interscience:New York, 1983. Table 1.  Summary of Electronic Absorption Data for LMoO(S-S) Complexes in Dichloroethane  E  max , cm - 1 (  , M - 1 cm - 1 )LMoO(bdt) LMoO(tdt) LMoO(qdt) 36 LMoO(edt)band 1 9 100 (360) 9 100 (490) 11 300 (170) 11 800 (160)band 2 13 100 (270) 13 000 (270) 13 700 (130) 15 500 (220)band 4 19 400 (sh,1220) 19 600 (sh,1320) 19 100 (1050) 20 000 (sh, 570) Figure 3.  Gaussian-resolved 293 K electronic absorption spectrum of LMoO(bdt) in dichloroethane (2.45  ×  10 - 4 M). The dashed linesrepresent the individual Gaussians used in the fit. Figure 4.  5 K electronic absorption (heavy line) and 4.86 K MCD(light line) spectra of LMoO(bdt) dispersed in poly(dimethylsiloxane). Table 2.  Calculated Oscillator Strengths of LMoO(bdt) and theRelationship between MCD and Electronic Absorption Bandsbandno.  E  (soln)max (cm - 1 )oscillatorstrength  E  (mull)max (cm - 1 )  E  (MCD)max (cm - 1 ) a MCDterm a 1 9 100 5.6 × 10 - 3 8 500 - - - - - - - -2 13 100 3.3 × 10 - 3 12 700 12 400  - C3 15 800 - - - - - - - - 15 700  + C4 19 400 1.6 × 10 - 2 19 200 19 300  + C5 22 100 1.7 × 10 - 2 21 500 21 000  + pseudo A6 25 100 9.2 × 10 - 2 24 600 24 300  + pseudo A a A positive pseudo A-term is a derivative shaped MCD feature withthe positive component at higher energy.  E  max  represents the point atwhich the pseudo A-term changes sign. Figure 5.  4.86 K MCD spectra of LMoO(bdt) (solid line) and LMoO-(tdt) (dotted line) dispersed in poly(dimethylsiloxane). Note the overallsimilarity of the spectral features. 1404  Inorganic Chemistry, Vol. 38, No. 7, 1999  Inscore et al.  been assigned as the Mo t O stretching vibration. Vibrationalstudies on related compounds possessing the  { Mo V t O } 3 + unitalso reveal the presence of a band in this region assignable asthe Mo t O stretch. 44 - 47 The IR spectra are also useful fordetecting very small quantities of LMoOCl 2  precursor complexthat may be present in the sample as a contaminant. No 961cm - 1 Mo t O stretch characteristic of LMoOCl 2  was observedin the IR spectra of the LMoO(S-S) complexes used in thisstudy. 32 Figure 6 shows the 514.5 nm Raman spectrum of LMoO-(bdt) in benzene. The solution spectrum contains only threeobservable vibrational modes between 250 and 1000 cm - 1 , withfrequencies of 362, 393, and 932 cm - 1 . Vibrational bands inthe 300 - 400 cm - 1 region of transition metal 1,2-ene-dithiolateshave been collectively assigned as Mo-S stretching vibrations. 48 - 50 A qualitative depolarization study was performed on LMoO-(bdt) in benzene using 496.5 nm excitation which yieldeddepolarization ratios of 0.40 (362 cm - 1 ), 0.22 (393 cm - 1 ), and0.01(932 cm - 1 ). Since the symmetry of the LMoO(S-S)complexes is very close to  C  s , the depolarization ratios for thesebands are characteristic of totally symmetric a ′  modes. Ramanspectra for LMoO(bdt) and LMoO(tdt) were also collected inthe solid-state at 140 K using laser excitation at wavelengthsbetween 528.7 and 457.9 nm. As was the case in solution, threevibrational bands were observed. These occured at 362, 393,and 931 cm - 1 for LMoO(bdt) and at 342, 376, and 926 cm - 1 for LMoO(tdt). Therefore, no significant ground-state vibrationalfrequency shifts occur between the solid and solution spectra,providing strong evidence that the structural integrity of thesecomplexes is maintained in solution. Resonance Raman Excitation Profiles.  Resonance Ramanexcitation profiles were collected in the solid state at 140 Kusing laser excitation wavelengths between 457.9 and 528.7 nm.This wavelength range encompasses the absorption envelopesof bands  4  and  5 . All three observed vibrational bands forLMoO(bdt) were found to be resonantly enhanced, and theresonance Raman profiles have been superimposed upon theGaussian resolved 5 K mull absorption spectrum in Figure 7.Extremely  selecti V  e  resonance Raman enhancement patterns areevident for excitation into band  4  (362 and 393 cm - 1 modes)and band  5  (931 cm - 1 mode). Interestingly, the resonanceRaman profile for the corresponding 926 cm - 1 mode of LMoO-(tdt) shows no high-energy turnover when pumping into band 5 . This is consistent with the low-temperature mull absorptionand MCD spectra of LMoO(tdt), which indicate that band  6  islowered in energy and overlaps band  5 . The result implies thatthe 931 cm - 1 mode of LMoO(bdt) is also resonantly enhancedwhen pumping into band  6 . Ab Initio Calculations.  Ab initio calculations on the modelethenedithiolate ( - SCH d CHS - ) fragment resulted in an isolatedset of four filled dithiolate orbitals that are primarily sulfur incharacter, and these are depicted in Figure 8. These are theligand wavefunctions which can energetically mix and formsymmetry-adapted linear combinations (SALC’s) with the dorbitals of appropriate symmetry localized on Mo. The calcula- (39) Chang, C. S. J.; Collison, D.; Mabbs, F. E.; Enemark, J. H.  Inorg.Chem.  1990 ,  29 , 2261 - 2267.(40) Chang, C. S. J.; Enemark, J. H.  Inorg. Chem.  1991 ,  30 , 683 - 688.(41) Nipales, N.; Westmoreland, T. D.  Inorg. Chem.  1995 ,  34 , 3374 - 3377.(42) Lincoln, S. E.; Loehr, T. M.  Inorg. Chem.  1990 ,  29 , 1907 - 1915.(43) Chang, C. S. J.; Pecci, T. J.; Carducci, M. D.; Enemark, J. H.  Inorg.Chem.  1993 ,  32 , 4106 - 4110.(44) Bradbury, J. R.; Mackay, M. F.; Wedd, A. G.  Aust. J. Chem.  1978 , 31 , 2423 - 2430.(45) Ellis, S. R.; Collison, D.; Garner, C. D.  J. Chem. Soc. Dalton Trans. 1989 , 413 - 417.(46) Burt, R. J.; Dilworth, J. R.; Leigh, G. J.; Zubieta, J. A.  J. Chem. Soc. Dalton Trans.  1982 , 2295 - 2298.(47) Ueyama, N.; Okamura, T.; Nakamura, A.  J. Am. Chem. Soc.  1992 , 114 , 8129 - 8137.(48) Spiro, T. (Ed.)  Molybdenum Enzymes ; John Wiley and Sons: NewYork, 1985.(49) Subramanian, P.; Burgmayer, S.; Richards, S.; Szalai, V.; Spiro, T.G.  Inorg. Chem.  1990 ,  29 , 3849 - 3853.(50) Oku, H.; Ueyama, N.; Nakamura, A.  Inorg. Chem.  1995 ,  34 , 3667 - 3676. Figure 6.  293 K resonance Raman spectrum of LMoO(bdt) in benzene.The spectrum was obtained with 514.5 nm excitation and the incidentlaser power at the sample was ∼ 75 mW. Unmarked bands are those of the solvent. Figure 7.  140 K solid-state resonance Raman excitation profiles forLMoO(bdt). The incident laser power measured at the sample was ∼ 50mW. The profiles are superimposed on the Gaussian-resolved 5 K mullabsorption spectrum of Figure 4. The dashed lines represent theindividual Gaussians used in the fit.  ν 1  (diamonds),  ν 3  (squares), ν 4  (circles). Figure 8.  Highest occupied molecular orbitals of the model 1,2-ene-dithiolate,  - SCH d CHS - . The small amplitudes of the wavefunctionon the dithiolate carbon atoms are not shown for clarity. Unique Bonding in Oxo-Molybdenum Dithiolates  Inorganic Chemistry, Vol. 38, No. 7, 1999  1405
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