Oxo-carboxylato-molybdenum ( VI ) complexes possessing dithiolene ligands related to the active site of type II DMSOR family molybdoenzymes †

Spectroscopic and kinetic studies indicate that oxo-carboxylatomolybdenum(VI) bis-dithiolene complexes, (MoO(p-X-OBz)L2), have been generated at low temperature as active site structural models for the type II class of pyranopterin molybdenum DMSOR family enzymes. A DFT analysis of low energy charge transfer bands shows that these complexes possess a Mo–Sdithiolene π-bonding interaction between the Mo(dxy) redox active molecular orbital and a cis S(pz) donor orbital located on one of the dithiolene ligands.

The vast majority of pyranopterin molybdenum enzymes catalyse oxygen atom transfer reactions between the substrate and solvent water coupled with proton and electron transfer. 1The dimethyl sulfoxide reductase (DMSOR) family of pyranopterin molybdenum enzymes is unique in that they possess two pyranopterin ene-1,2-dithiolate ligands bound to the Mo ion. 1 The active site Mo centre is the locus of the oxygen atom transfer reactivity and can adopt desoxomolybdenum(IV) and monooxomolybdenum(VI) structures during the catalytic cycle. 1,2Phylogenic analysis and protein sequence alignments of the metalbinding regions have been used to further subdivide DMSOR family enzymes into three types: I, II and III (Fig. 1). 3 Type I enzymes possess a Cys or SeCys residue that coordinates to the molybdenum centre, 4 while type II and type III enzymes have a metal center coordinated by an Asp and a Ser residue, respectively. 2,57][8] Specifically, these researchers have synthesised and structurally characterised a number of desoxomolybdenum(IV) complexes employing 1,2-dimethylethylene-1,2-dithiolate (S 2 C 2 Me 2 ) together with RS − , RSe − , RCOO − , and RO − ligands (X) 6 that are structurally related to the molybdenum(IV) active site structures of DMSOR type I, II and III enzymes. 7However, the corresponding monooxomolybdenum(VI) complexes, [Mo VI O(L)(S 2 C 2 Me 2 ) 2 ] − , have proved too unstable for full characterisation due to an auto-redox reaction between the Mo VI ion and the monodentate ligand L that results in the formation of the five-coordinate [Mo V O(S 2 C 2 Me 2 ) 2 ] − complex. 7,8The only crystallographically characterised example of a [ , where the six-coordinate structure appears to be stabilised by the presence of electron-withdrawing methoxycarbonyl (-COOMe) groups on the dithiolene. 9Mo VI O(OSi i Pr 3 )-(L COOMe ) 2 best represents a synthetic analogue of the DMSOR type III enzyme active sites.Recently, we prepared and characterised oxosulfido-and oxoselenido-molybdenum(VI) complexes at low temperature as active site models for the xanthine oxidase family of pyranopterin molybdenum enzymes. 10Herein, we report the successful application of low temperature techniques to generate and characterise new bis-(ene-1,2-dithiolate)oxocarboxylatomolybdenum(VI) complexes, which can be regarded as structural models for oxidised

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View Journal | View Issue members of type II DMSOR family enzymes.Additionally, we provide an initial electronic structure description of these bis-(ene-1,2-dithiolate)oxocarboxylatemolybdenum(VI) complexes using DFT calculations.The designation and abbreviation of the complex structures are given in Chart S1. † Initial attempts to prepare the oxocarboxylatomolybdenum(VI) complex ) utilised an oxo-transfer reaction from the tertiary amine oxide (Me 3 NO) to the benzoatomolybdenum(IV) complex, Mo IV ( p-H-OBz)L 2 , at room temperature under an inert Ar atmosphere.However, the grey coloured oxomolyb- − with tertiary amine oxides or sulfoxides. 8We attempted to observe the formation of (Mo VI O( p-H-OBz)L 2 at low temperatures, but the reaction between Mo IV ( p-H-OBz)L 2 and tertiary amine oxides does not proceed to a significant extent below −40 °C.Next, the reaction of the five-coordinate oxomolybdenum(VI) complex, Mo VI OL 2 (synthesis in ESI †), 11 with the benzoate anion, Et 4 N( p-H-OBz), was examined in C 2 H 5 CN under an Ar atmosphere.At room temperature, this reaction also yielded the oxomolybdenum(V) complex (Mo V OL 2 ) (Fig. S1 †), but at low temperature (−60 °C) the reaction yielded a deep-green EPR silent product.Fig. 2 shows the observed spectral changes upon addition of p-H-OBz − to Mo VI OL 2 in C 2 H 5 CN at −60 °C.Here, the absorption band at 395 nm due to Mo VI OL 2 decreases with the concomitant appearance of new absorption bands at 807 nm (band 1) and 597 nm (band 2).Tight isosbestic points are observed at 336, 367 and 445 nm.Titration plots clearly indicate that the stoichiometry of Mo VI OL to p-H-OBz − is 1 : 1 (Fig. 2, inset).The final spectrum is very similar to that of [Mo VI O(OSi i Pr 3 )-(S 2 C 2 (COOMe) 2 ) 2 ] − , 9 consistent with the formation of a sixcoordinate oxocarboxylatomolybdenum(VI) complex, [Mo VI and Mo VI O( p-Cl-OBz)L 2 possess similar spectral features to Mo VI O( p-H-OBz)L 2 and were generated in a similar manner using Et 4 N( p-OMe-OBz) and Et 4 N( p-Cl-OBz), respectively (Fig. S2 †).The λ max for band 1 shifts to lower energy as the substituent X changes from electron withdrawing to electron donating (X = Cl, 793 nm; X = H, 807 nm; X = OMe, 814 nm).This strongly supports the direct coordination of the benzoate ligand to the molybdenum(VI) centre.When warmed to room temperature, the UV-vis and EPR spectra converted to those of Mo V OL 2 .
Cyclic voltammograms of 0.1 M n Bu 4 NPF 6 -C 2 H 5 CN solutions containing Mo VI OL 2 and Et 4 N( p-X-OBz) (X = Cl, H, OMe) in a 1 : 1 ratio were measured at −60 °C and yielded one irreversible reduction wave below −1 V vs. SCE.The reduction peak potential is observed to shift in a negative direction (Cl, E pc = −1.08V; H, E pc = −1.16;OMe, E pc = −1.18V vs. SCE) as the pK a value of the p-substituted benzoic acid increases (3.99 for X = Cl, 4.20 for X = H and 4.50 for X = OMe).This result provides support for the coordination of the p-X-OBz anion to the Mo center of Mo VI OL 2 since the electron-donating substituent increases the basicity of the benzoate anion and enhances its ability to coordinate to Mo VI .The CSI-mass spectrum of a C 2 H 5 CN solution containing Mo VI OL 2 and 1 equiv. of Et In order to obtain information about the mechanism of Mo VI O( p-X-OBz)L 2 formation, a low temperature kinetic study was performed.The spectral changes observed upon addition of the benzoate ligand to Mo VI OL 2 were observed to be biphasic.The data show a rapid disappearance of the Mo VI OL 2 395 nm band and the appearance of a new absorption band at 360 nm due to intermediate A that converts to Mo VI O(p-X-OBz)-L 2 with characteristic absorption bands at 597 and 807 nm (Fig. S4a and 4b †).Although the time course of the first step was too fast to be followed accurately, the conversion of intermediate A to Mo VI O( p-X-OBz)L 2 obeys first-order kinetics.It should be noted that the observed first-order rate constant, k obs , was independent of the concentration of Et 4 N( p-H-OBz) (Fig. S5 †).The formation of Mo VI O( p-Cl-OBz)L 2 and Mo VI O-( p-OMe-OBz)L 2 displayed kinetic behavior similar to Mo VI O-( p-H-OBz)L 2 with k obs increasing as the electron-withdrawing character of X increases: 65.8 × 10 −3 s −1 for X = Cl, 7.2 × 10 −3 s −1 for X = H and 1.4 × 10 −3 s −1 for X = OMe.These observations suggest that the first step is the association (coordination) of the benzoate anion with the Mo center of Mo VI OL 2 , giving intermediate A, and the second step is the intramolecular rearrangement of intermediate A to the product (Scheme 1).DFT calculations support an idealised C 2v structure with respect to the two S 2 C 2 C 4 H 8 ligands for intermediate A that subsequently rearranges via a Ray-Dutt type twist 12 to form a more stable product with a distorted octahedral geometry (Fig. 3).
The DFT optimised structure of Mo VI O( p-H-OBz)L 2 possesses a distorted octahedral coordination environment that is similar to the related Mo VI O(OSi i Pr 3 )(L COOMe ) 2 (vide supra; Fig. 3), which has been previously characterised by X-ray crystallography. 9 1).This Mo-S π* bonding description is similar to what we observed in the related complex Mo VI O(OSi i Pr 3 )(L COOMe ) 2 , 9 and this derives from the ∼180°O oxo -Mo-S-C dihedral angle involving the cis S of dithiolene B.

Conclusions
New bis(ene-1,2-dithiolato)oxocarboxylatomolybdenum(VI) complexes have been synthesized as active site analogues of type II DMSOR family enzymes.Low temperature kinetic and spectroscopic analyses, in conjunction with DFT calculations, have been used to understand their formation and electronic structure.Provided DMSOR ox possesses a hexacoordinate geometry; these results suggest that the ancillary O asp ligand likely functions to fine tune the redox potential of the Mo ion.Interestingly, a hexacoordinate geometry also allows for a specific pyranopterin dithiolene to couple the active site into long-range superexchange pathways for electron transfer regeneration of the catalytically relevant Mo(IV) site.
This work was partly supported by grants (no.23350027, 24108725 and 2410915 to H.S. and no.22105007 to S.I.).M.L.K. acknowledges the National Institutes of Health (GM 057378) for financial support.The authors also thank Dr Kei Ohkubo and Prof. Shunichi Fukuzumi of Osaka University for their help in collecting the EPR spectrum.a Values in parentheses are the S contribution of the dithiolene.

Fig. 1
Fig.1The active site structures of oxidized type I, II and III enzymes.
4 N-( p-OMe-OBz) showed a peak cluster attributable to [Mo-( p-OMe-OBz)(S 2 C 2 C 4 H 8 ) 2 ] − at m/z = 537 at −60 °C (Fig. S3 †).Since the UV-vis spectrum of the C 2 H 5 CN solution of the product shown in Fig. 2 is different from that of [Mo IV ( p-OMe-OBz)(S 2 C 2 C 4 H 8 ) 2 ] − synthesised separately from [Mo IV -(S 2 C 2 C 4 H 8 ) 2 ] and p-OMe-OBz − , this peak cluster is likely to be a fragment of Mo VI O( p-OMe-OBz)L 2 .Therefore, we conclude that Mo VI O( p-H-OBz)L 2 and its para substituted derivatives are formed at low temperature by coordination of the benzoates to the molybdenum(VI) centre of Mo VI OL 2 .
The computations indicate a slightly larger S1-S2-S3-S4 dithiolene dihedral angle for Mo VI O( p-H-OBz)L 2 of 126°compared with a 108°dihedral angle for structurally characterised Mo VI O(OSi i Pr 3 )(L COOMe ) 2 .As was observed for Mo VI O(OSi i Pr 3 )(L COOMe ) 2 , the Mo-S4 bond distance (2.55 Å) is elongated when compared to the three Mo-S bonds (mean 2.42 Å) as a result of a strong trans influence due to the MouO bond.Geometry optimisations indicate that Mo VI O( p-Cl-OBz)-L 2 and Mo VI O( p-OMe-OBz)L 2 possess S2-S1-S3-S4 dihedral angles, and Mo-S, MouO oxo and Mo-O OBz bond distances, which are nearly identical to those of Mo VI O( p-H-OBz)L 2 .The computed structures for the Mo VI O( p-X-OBz)L 2 complexes display ∼172°O oxo -Mo-S3-C dihedral angles, allowing for strong Mo d xy -S dithiolene π-bonding involving a single S3 donor on dithiolene B. This is supported by bonding calculations that show a LUMO wavefunction which possesses a strong Mo-S dithiolene π* bonding interaction between the Mo(d xy ) orbital and the cis S3( p z ) donor orbital located on dithiolene (B) (Table Scheme 1 Proposed mechanism for formation of Mo VI O( p-X-OBz)L.

Table 1
Molecular orbital compositions for Mo VI O( p-H-OBz)L 2