IV (E = O, S) core units

Inorganica Chimica Acta 363 (2010) 1126–1132

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Isovalent and mixed-valent molybdenum complexes containing MoV(l-E)(l-S2)MoV/IV (E = O, S) core units Craig Gourlay a, Michelle K. Taylor a,b, Paul D. Smith a,1, Charles G. Young a,* a b

School of Chemistry, University of Melbourne, Victoria 3010, Australia Bio21 Molecular Science and Biotechnology Institute, University of Melbourne, Victoria 3010, Australia

a r t i c l e

i n f o

Article history: Received 26 June 2009 Accepted 13 September 2009 Available online 19 September 2009 Dedicated to Prof. Jonathan R. Dilworth, on the occasion of his 65th birthday and in recognition of his contributions to chemistry. Keywords: Scorpionate complexes l-Sulfido-l-disulfido complexes Molybdenum complexes Mixed-valent complexes Dinuclear complexes Supramolecular assembly Crystal structure

a b s t r a c t The reactions of TpiPrMoO(SR)(NCMe) (TpiPr = hydrotris(3-isopropylpyrazolyl)borate) with propylene sulfide in toluene result in the formation of the diamagnetic, isovalent Mo(V) complex, [TpiPrMoVO]2(l-S)(l-S2). This complex and its previously reported l-oxo analog, [TpiPrMoVO]2(l-O)(l-S2), react with cobaltocene to produce one-electron-reduced, mixed-valent complexes, [CoCp2][{TpiPrMoIV,VO}2(l-E) (l-S2)] (E = S or O, respectively). All complexes have been isolated and characterized by microanalysis, mass spectrometry, IR and 1H NMR or EPR spectroscopies, and X-ray crystallography. Neutral [TpiPrMoV O]2(l-S)(l-S2) exhibits a pseudo-C2 symmetric structure, with distorted octahedral anti oxo-Mo(IV) centers coordinated by TpiPr and linked by l-sulfido and l-disulfido ligands. A similar structure is adopted by the anion in mixed-valent [CoCp2][{TpiPrMoIV,VO}2(l-S)(l-S2)]; this compound adopts a hexagonal, supramolecular structure with columns of tight ion-pairs with ðl-S2 Þ    CoCpþ 2 interactions, interconnected through weaker ðl-SÞ    CoCpþ 2 contacts to three neighboring columns. The structure contains large interstitial voids filled with lattice solvent molecules. EPR investigation of the mixed-valent complexes gave rise to unusually broad signals with no evident hyperfine splitting. The synthesis and characterization of a number of cis-dioxo-Mo(VI) precursors are also reported. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Recently, we reported the synthesis of novel oxosulfido-Mo(VI) complexes, TpiPrMoOS(OAr), where TpiPr is the sterically bulky hydrotris(3-isopropylpyrazolyl)borate ligand and OAr is a substituted phenolate or naphtholate co-ligand [1,2]. These complexes are of interest as spectroscopic and functional models of the molybdenum hydroxylases, a family of enzymes responsible for the hydroxylation of purines, pyrimidines and aldehydes and the translocation of aromatic hydroxyl groups [3,4]. The enzymes contain a square pyramidal, oxidized active site, viz., [(MPT) MoVIOS(OH)], and form a number of Mo(V) states during turnover, e.g., the very rapid form, formulated as [(MPT)MoVOS(OSub)]2 (MPT = molybdopterin, Sub = substrate) [3,4]. Spectroscopic and computational studies have revealed a large degree of covalency in the Mo@S bond in oxosulfido-Mo(VI/V) complexes [5], consistent with parallel findings for various enzyme states [6]. Very recently, we extended our oxosulfido-Mo chemistry to

* Corresponding author. Tel.: +61 3 8344 6515; fax: +61 3 9347 5180. E-mail address: [email protected] (C.G. Young). 1 Present address: Division of Chemistry and Materials, School of Biology, Chemistry and Health Science, Manchester Metropolitan University, Manchester M1 5GD, UK. 0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2009.09.033

MoO(l-S)Cu complexes [7] that model the (MCD)MoO(OH) (l-S)Cu(Scys) (MCD = molybdopterin cytosine dinucleotide) active site of the CO dehydrogenase from Oligotropha carboxidovorans [8,9]. The TpiPrMoOS(OAr) complexes have a rich and emerging chemistry. For example, they participate in monomer–dimer equilibria [1,2] and phenolate C–H activation processes [2], and are reduced to Mo(V) analogs amenable to cupration at the sulfido ligand [7]. Under aerobic conditions, they form the thermodynamically stable dinuclear Mo(V) complex, [TpiPrMoO]2(l-O)(l-S2); this complex may be produced by hydrolysis of the disulfido-bridged dimer or in a step-wise hydrolysis involving intermediates such as TpiPrMoOS(OH) (Scheme 1) [2]. To date, our successful development of oxosulfido-Mo chemistry has been restricted to the use of redox-inactive, O-donor, phenolate co-ligands (OAr). Oxosulfido-Mo(VI) complexes containing S-donor co-ligands have been reported but these are generally stabilized by intramolecular S  S interactions. Examples include TpxMoOS(S2PR2) (R = i Pr, Ph; Tpx = hydrotris(3,5-dimethylpyrazolyl)borate (Tp*) [10], TpiPr [11]), complexes that are stabilized by a partial S  S bond between the sulfido ligand and the uncoordinated dithiophosphinate sulfur atom. The Tp* complexes [10,12] participate in counter-intuitive redox reactions as a consequence of the facile redox interplay of Mo and S centers; this behavior is also a feature of other systems

C. Gourlay et al. / Inorganica Chimica Acta 363 (2010) 1126–1132

2 H

S

Mo

N

N B

of mixed-valent Mo complex. The supramolecular structure of [CoCp2][{TpiPrMoO}2(l-S)(l-S2)] is a notable facet of the chemistry reported.

O

N N

ArO

O

O

TpiPrMo

N

S

S

OAr

R

N

MoTpiPr O

H2O, –HOAr O

O S

+

TpiPrMo

S

–HOAr

O TpiPrMo

OH

OAr

S

S O

MoTpiPr O

Scheme 1. Possible modes of formation of [TpiPrMoO]2(l-O)(l-S2) [2].

[13,14]. The propensity of Mo/S species to undergo these reactions has plagued efforts to synthesize monomeric oxosulfido-Mo(VI) dithiolene complexes, dimeric species being the common outcome of such campaigns. A deeper understanding of the metal-induced reactions involving S-donor ligands and the factors influencing the course of such reactions may provide key insights to the eventual stabilization and isolation of more accurate models for the Mo enzymes. The present work emerged from attempts to prepare S-donor, thiolate complexes of the type TpiPrMoOS(SR) (R = alkyl, aryl). Here, reactions involving the redox-active thiolate ligands led to the formation of [TpiPrMoO]2(l-S)(l-S2), the l-sulfido analog of [TpiPrMoO]2(l-O)(l-S2) (Scheme 1) [2]. The study expanded into mixed-valent complexes when [TpiPrMoO]2(l-E)(l-S2) (E = O, S) were observed to undergo electrochemically-reversible, one-electron reductions and [CoCp2][{TpiPrMoO}2(l-O)(l-S2)] was isolated as a product of the chemical reduction of TpiPrMoOS(OAr) using cobaltocene. Optimized syntheses for [TpiPrMoO]2(l-S)(l-S2) and [CoCp2][{TpiPrMoO}2(l-E)(l-S2)] were subsequently developed, allowing the complexes to be fully characterized. Scheme 2 summarizes the chemical reactions and dinuclear products reported herein. The synthesis and properties of a number of new dioxoMo(VI) thiolate complexes, which are precursors to the oxo-Mo(IV) acetonitrile species in Scheme 2, are also reported. Dinuclear MoV2(l-O)(l-S2) complexes are rare [2,15,16] and MoV2(l-S)(l-S2) species are currently unknown. Moreover, there are no reports of mixed-valent complexes containing Mo2(lE)(l-S2) cores. Known dinuclear Mo(IV,V) complexes include the l-oxo-dithiocarbamate complexes, [Mo2O(S2CNR2)6]X ðR ¼ Me;   Et; X ¼ BF 4 ; PF6 ; ClO4 Þ [17], (H5O2)[Mo2O3(S2CNR2)4] [18] and [Mo2O2(S2CNR2)4]PF6 [19], and the dimethyl sulfide complex, (Me2S)2Cl2MoIV(l-S2)(l-S)MoVCl3(SMe2) [20]. Other less well characterized species have been generated in situ by electrochemical and radiolytic methods [21]. This paper reports the first example of a complex containing an MoV2(l-S)(l-S2) core and a new class

O 2

C

N

TpiPrMo

Me

O TpiPrMo

S

O

S

S S

MoTpiPr O

CoCp2

–3C3H6, –RSSR, –2NCMe

O TpiPrMo

S

S

SR 3

2. Experimental 2.1. Materials and methods

H2O, –2HOAr

TpiPrMo

1127

O MoTpiPr O

CoCp2

CoCp2 TpiPrMo

S

S E

(E = O, S) Scheme 2. Synthesis of complexes reported herein.

MoTpiPr O

Unless, specified, all reactions were performed under an atmosphere of dinitrogen using dried, deoxygenated solvents and standard Schlenk techniques. The complexes TpiPrMoO(SiPr)(NCMe) (and related derivatives) [22] and [TpiPrMoO]2(l-O)(l-S2) [2] were prepared according to literature methods or adaptions thereof. Cobaltocene was obtained from Aldrich Chemical Co. and purified by sublimation before use. Solid-state (KBr disk) IR spectra were recorded on a Bio-rad FTS 165 FTIR spectrophotometer and NMR spectra were obtained using a Varian FT Unity 400 MHz spectrometer. 1H and 19F NMR spectra were referenced to residual chloroform (dH = 7.24) and external CCl3F, respectively. EPR spectra were recorded on a Bruker FT ECS-106 spectrometer at X-band frequencies using 1,1-diphenyl-1,2-picrylhydrazyl as reference. Electrospray ionization mass spectrometric (ESI-MS) experiments were carried out in positive-ion mode using a Micromass Quattro II mass spectrometer using samples dissolved in MeCN or MeCN/ CH2Cl2 or MeCN/MeOH mixtures. Cyclic voltammograms were recorded using a 2 mm glassy carbon working electrode, platinum counter electrode and a freshly prepared double-jacketed Ag/ AgNO3 reference electrode (10 mM AgNO3 in MeCN with 0.1 M NBun4 PF6 and clean silver wire), connected to an Autolab Potentiostat operated by the General Purpose Electrochemical System software (version 4.9). Samples were prepared as 1–2 mM solutions in MeCN with 0.1 M NBun4 PF6 as supporting electrolyte and scan rates over the range 10–500 mV s1. Potentials were referenced against the ferrocene couple, Fc+/Fc, and are reported relative to SCE. The Fc+/Fc couple was set to the reported value of +0.400 V vs. SCE for acetonitrile/0.1 M NBun4 PF6 solutions [23]. Microanalyses were performed by Atlantic Microlab Inc., Norcross, GA, USA. 2.2. Syntheses 2.2.1. Dioxo-Mo(VI) complexes The complexes, TpiPrMoO2(SR), were prepared in the manner described previously for other derivatives [22,24,25]. A solution of TpiPrMoO2Cl (1.50 g, 2.98 mmol) in dichloromethane (30 mL) was treated with triethylamine (1.2 mL, 7.4 mmol), followed by thiol (8.0 mmol). The yellow solution became dark red–brown and was left to stir until deletion of starting material (as indicated by thin layer chromatography) was achieved. The mixture was reduced to dryness and the residue was column chromatographed on silica gel using 3:2 dichloromethane/hexane as eluent. The first red band was collected and evaporated to dryness. The residue was then treated with hexane (5 mL) and stirred and cooled to ca. 0 °C, to yield brown crystals. The product was collected by filtration, washed with cold hexane and dried in vacuo. R = CHPh2. Yield 48%. Anal. Calc. for C31H39BMoN6O2S: C, 55.86; H, 5.89; N, 12.61; S, 4.81. Found: C, 55.64; H, 5.83; N, 12.81; S, 5.02%. IR (KBr, cm1): m(BH) 2479, m(CN) 1504 s, m(MoO2) 922 s and 891 s. 1H NMR (CDCl3): d 1.00, 1.02, 1.20 (each d, 6H, J = 6.8 Hz, 2  CH3); 3.91 (sept, 1H, J = 6.8 Hz, CH(CH3)2), 3.92 (sept, 2H, J = 6.8 Hz, 2  CH(CH3)2); 5.99 (s, 1H, CHPh2); 6.03 (d, 2H, J = 2.4 Hz, 2  4-CH), 6.05 (d, 1H, J = 2.4 Hz, 4-CH); 7.2–7.6 (m, 10H, 2  Ph); 7.54 (d, 2H, J = 2.4 Hz, 2  5-CH), 7.56 (d, 1H, J = 2.4 Hz, 5-CH). R = C6H4F-4. Yield 52%. Anal. Calc. for C24H32BFMoN6O2S: C, 48.50; H, 5.43; N, 14.14; S, 5.39. Found: C, 48.33; H, 5.32; N, 14.48; S, 5.21%. IR (KBr, cm1): m(BH) 2509 m, m(CN) 1507 s,

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m(MoO2) 927 s and 896 s. 1H NMR (CDCl3): d 1.14, 1.19, 1.28 (each d, 6H, J = 6.8 Hz, 2  CH3); 3.91 (sept, 1H, J = 6.8 Hz, CH(CH3)2), 4.14 (sept, 2H, J = 6.8 Hz, 2  CH(CH3)2); 6.08 (d, 1H, J = 2.4 Hz, 4-CH), 6.13 (d, 2H, J = 2.4 Hz, 2  4-CH); 7.02 (t, 2H, JHH = 9.0 Hz, JHF = 9.0 Hz, 3,5-CH of SC6H4F-4), 7.59 (dd, 2H, JHH = 9.0 Hz, JHF = 5.4 Hz, 2,6-CH of SC6H4F-4); 7.60 (d, 2H, J = 2.4 Hz, 2  5CH), 7.62 (d, 1H, J = 2.4 Hz, 5-CH). R = C6H4iPr-2. Yield 68%. Anal. Calc. for C27H39BMoN6O2S: C, 52.44; H, 6.36; N, 13.59; S, 5.18. Found: C, 52.34; H, 6.30; N, 13.78; S, 5.09%. IR (KBr, cm1): m(BH) 2509 m, m(CN) 1508 s, m(MoO2) 931 s and 896 s. 1H NMR (CDCl3): d 1.10, 1.16, 1.20, 1.22 (each d, 6H, J = 6.8 Hz, 2  CH3); 3.63 (sept, 1H, J = 6.8 Hz, CH(CH3)2), 4.02 (sept, 1H, J = 6.8 Hz, CH(CH3)2), 4.04 (sept, 2H, J = 6.8 Hz, 2  CH(CH3)2); 6.09 (d, 1H, J = 2.4 Hz, 4-CH), 6.12 (d, 2H, J = 2.4 Hz, 2  4-CH); 7.14 (t, 1H), 7.24 (d, 1H), 7.27 (t, 1H), 7.96 (d, 1H) (all J = 7.2 Hz, SC6H4iPr-2); 7.61 (d, 2H, J = 2.4 Hz, 2  5-CH), 7.62 (d, 1H, J = 2.4 Hz, 5-CH). R = C6H4tBu-2. Yield 46%. Anal. Calc. for C28H40BMoN6O2S: C, 53.26; H, 6.39; N, 13.31; S, 5.08. Found: C, 53.34; H, 6.31; N, 13.43; S, 5.09%. IR (KBr, cm1): m(BH) 2504 m, m(CN) 1506 s, m(MoO2) 923 s and 897 s. 1H NMR (CDCl3): d 1.06, 1.20, 1.27 (each d, 6H, J = 6.8 Hz, 2  CH3); 1.42 (s, 9H, C(CH3)3); 3.90 (sept, 1H, J = 6.8 Hz, CH(CH3)2), 3.94 (sept, 2H, J = 6.8 Hz, 2  CH(CH3)2); 6.07 (d, 1H, J = 2.4 Hz, 4-CH), 6.11 (d, 2H, J = 2.4 Hz, 2  4-CH); 7.44 (t, 1H), 7.24 (m, 2H), 7.27 (t, 1H), 8.09 (d, 1H) (all J ca. 7.2 Hz, SC6H4tBu-2); 7.54 (d, 2H, J = 2.4 Hz, 2  5-CH), 7.59 (d, 1H, J = 2.4 Hz, 5-CH). R = C6H4Br-2. Yield 39%. Anal. Calc. for C24H32BBrMoN6O2S: C, 43.99; H, 4.92; N, 12.83; S, 4.89. Found: C, 44.02; H, 4.70; N, 12.78; S, 4.76%. IR (KBr, cm1): m(BH) 2506 m, m(CN) 1506 s, m(MoO2) 923 s and 898 s. 1H NMR (CDCl3): d 1.09, 1.23, 1.24 (each d, 6H, J = 6.8 Hz, 2  CH3); 3.96 (sept, 2H, J = 6.8 Hz, 2  CH(CH3)2), 3.97 (sept, 1H, J = 6.8 Hz, CH(CH3)2); 6.10 (d, 1H, J = 2.4 Hz, 4-CH), 6.14 (d, 2H, J = 2.4 Hz, 2  4-CH); 6.98 (t, 1H), 7.39 (t, 1H), 7.56 (d, 1H), 8.08 (d, 1H) (all J = 7.2 Hz, SC6H4Br-2); 7.62 (d, 2H, J = 2.4 Hz, 2  5-CH), 7.64 (d, 1H, J = 2.4 Hz, 5-CH). R = C10H7-2 (2-naphthyl). Yield 58%. Anal. Calc. for C28H35BMoN6O2S: C, 53.69; H, 5.63; N, 13.42; S, 5.12. Found: C, 53.64; H, 5.38; N, 13.60; S, 5.09%. IR (KBr, cm1): m(BH) 2497 m, m(CN) 1507 s, m(MoO2) 931 s and 901 s. 1H NMR (CDCl3): d 1.12, 1.21, 1.29 (each d, 6H, J = 6.8 Hz, 2  CH3); 3.97 (sept, 1H, J = 6.8 Hz, CH(CH3)2), 4.20 (sept, 2H, J = 6.8 Hz, 2  CH(CH3)2); 6.09 (d, 1H, J = 2.4 Hz, 4-CH), 6.14 (d, 2H, J = 2.4 Hz, 2  4-CH); 7.4–8.1 (m, 7H, SC10H7-2); 7.61 (d, 2H, J = 2.4 Hz, 2  5-CH), 7.64 (d, 1H, J = 2.4 Hz, 5-CH). 2.2.2. [TpiPrMoO]2(l-S)(l-S2) Propylene sulfide (0.68 mL, 8.25 mmol) was added to a blue solution of TpiPrMoO(SiPr)(NCMe) (0.95 g, 1.65 mmol) in toluene (15 mL). The solution turned brown within 5 min and was stirred at room temperature for 1 h. The solvent was removed under vacuum to give a brown oil that was column chromatographed (under anaerobic conditions) using silica gel and 3:2 deoxygenated CH2Cl2/hexane as eluent. The main brown fraction was collected and evaporated to dryness in vacuo. The brown residue was dissolved in hexane (ca. 5 mL) and the solution was stored at 30 °C overnight. The resulting brown powder was collected by filtration and dried under vacuum. Slight evaporation of the filtrate and storage at 30 °C yielded crystals suitable for X-ray diffraction. Typical total yield 0.5 g (62%). Anal. Calc. for C36H56B2Mo2N12O2S3: C, 43.30; H, 5.65; N, 16.83; S, 9.63. Found: C, 43.37; H, 5.84; N, 16.90; S, 9.49%. IR (KBr, cm1): 2967 s, 2925 m, 2866 m, m(BH) 2497 m and 2462 m, m(CN) 1509 s, 1458 w, 1402 m, 1396 m, 1384 m, 1363 m, 1287 w, 1248 w, 1193 s, 1104 w, 1069 m, 1047 s, m(Mo@O) 923 s, 794 w, 776 m, 733 m. 1H NMR (CDCl3): d 1.06 (6H), 1.15 (6H), 1.21 (12H), 1.30 (6H), 1.32

(6H) (each d, J = 6.8 Hz, total 12  CH(CH3)2); 3.78, 4.54, 4.63 (each sept, 2H, J = 6.8 Hz, 2  CH(CH3)2); 6.13 (d, br, 4H, 4  4-CH), 6.20 (d, 2H, J = 2.4 Hz, 2  4-CH); 7.56, 7.66, 7.68 (each d, 2H, J = 2.4 Hz, 2  5-CH). MS: m/z 1021 [M+Na]+, 509 [TpiPrMoOSH+Na]+, 531 [TpiPrMoOS2+Na]+. 2.2.3. [CoCp2][{TpiPrMoO}2(l-S)(l-S2)] A solution of [TpiPrMoO]2(l-S)(l-S2) (0.42 g, 0.42 mmol) in toluene (3–4 mL) was treated with a solution of CoCp2 (0.1 g, 0.513 mmol) in toluene (3–4 mL), to produce an immediate dark brown precipitate. The mixture was stirred for 10 min and the solvent volume was reduced by half. The brown product was collected by filtration, washed with hexane and dried under vacuum. Yield 0.4 g (80%). The product was recrystallized by slowly cooling a hot solution of the complex in tetrahydrofuran to room temperature. Crystals suitable for X-ray diffraction were obtained in this fashion. The sample for elemental analysis was ground to a fine powder and dried at 60 °C under vacuum for 48 h. Anal. Calc. for C46H66B2CoMo2N12O2S3: C, 46.52; H, 5.60; N, 14.15; S, 8.10. Found: C, 46.73; H, 5.63; N, 13.65; S, 7.99%. IR (KBr, cm1): 3109 m, 2964 s, 2922 m, 2865 m, m(BH) 2476 m and 2444 m, 1635 w, br, m(CN) 1508 s 1489 m, 1461 w, 1415 m, 1397 s, 1384 s, 1362 m 1293 w, 1246 w, 1197 s, 1105 m, 1069 m, 1044 s, 1011 w, 930 m, sh, m(MoO) 920 m, 859 w, 815 w, 792 m, 775 m, 736 s, 458 m. EPR (CH2Cl2, 298 K): broad signal = 1.987. EPR (10:1 THF/MeCN, 77 K): g1 2.126, g2 1.946, g3 1.900. 2.2.4. [CoCp2][{TpiPrMoO}2(l-O)(l-S2)] The complex was prepared from [TpiPrMoO]2(l-O)(l-S2) (0.4 g, 0.41 mmol) and CoCp2 (0.1 g, 0.513 mmol) using the method described above for the sulfido analog. Yield 0.2 g (40%). Crystals were obtained by slow cooling of a hot tetrahydrofuran solution of the complex. The sample for elemental analysis was ground to a fine powder and dried at 60 °C under vacuum for 48 h. Anal. Calc. for C46H66B2CoMo2N12O3S2: C, 47.15; H, 5.69; N, 14.35; S, 5.47. Found: C, 47.24; H, 5.84; N, 14.12; S, 5.37%. IR (KBr, cm1): 2964 s, 2926 m, 2869 m, m(BH) 2484 m and 2446 m, 1636 w,br, m(CN) 1506 s, 1491 m, 1462 w, 1415 m, 1398 s, 1385 s, 1363 m 1295 w, 1282 w, 1261 w, 1247 w, 1228 w, 1197 s, 1185 s, 1106 m, 1069 m, 1049 s, 1023 w, 1014 w, m(MoO) 942 s and 920 m, 860 w, 816 w, 794 m, 776 m, 749 w, 734 s, 697 w, 659 w, 631 w, 501 w, 462 m. EPR (10:1 THF: MeCN, 298 K): broad signal = 1.996. EPR (10:1 THF:MeCN, 77 K): g1 2.172, g2 1.944, g3 1.884. 2.3. X-ray crystallography Diffraction data were collected at 130 K using a Bruker CCD diffractometer with an Mo Ka (0.71073 Å) radiation source. Data were collected to 2hmax = 55° for each structure. Cell parameters were acquired by the SMART software package and data reductions were performed using SAINT [26]. Structures were solved by direct methods (SHELXS-97) [27] and refined using full-matrix leastsquares on F2 (SHELXL-97) [28]. Molecular diagrams were generated using ORTEP 3 and CRYSTAL MAKER software [29]. Crystal data are provided in Table 1. Selected distances and angles are presented in Table 2. For [TpiPrMoO]2(l-S)(l-S2), the single molecule in the asymmetric unit was refined using anisotropic thermal parameters for all non-hydrogen atoms. Examination of the structure with PLATON [30] showed that it contained large intermolecular voids that were occupied by disordered hexane molecules. The disorder in the solvent molecules prevented accurate modeling. Therefore, the solvent contribution was treated using the SQUEEZE procedure of PLATON [30], from which an estimate of the solvent content was obtained on the basis of the volume of the voids and the approximate number of electrons contained therein. This was estimated to be 3

1129

C. Gourlay et al. / Inorganica Chimica Acta 363 (2010) 1126–1132 Table 1 Crystallographic data.

a b

Parameter

[TpiPrMoO]2(l-S)(l-S2) 3(C6H14)

[CoCp2][{TpiPrMoO}2(l-S)(l-S2)] 3.33(C4H8O)

Formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z T (K) q (g cm3) l (cm1) Data Unique data R1 [I > 2r(I)]a wR2 (F2, all data)b Goodness-of-fit (GOF)

C54H98B2Mo2N12O2S3 1257.12 monoclinic P21/c 18.0404(20) 16.3876(18) 22.4409(25) 90 107.481(2) 90 6328.0(12) 4 130(2) 1.320 5.43 38874 14271 0.0515 0.0994

C59.33H92.67B2Co Mo2N12O5.33S3 1428.06 trigonal  R3c 35.4380(19) 35.4380(19) 30.1001(32) 90 90 120 32737(4) 18 298(2) 1.304 7.02 64060 8219 0.0509 0.1191

0.810

0.791

contained very large voids occupied by disordered tetrahydrofuran molecules that could not be adequately modeled. Therefore, the solvent contribution was treated using the SQUEEZE procedure of PLATON [30]. The solvent content was estimated to be 3.33 THF molecules per cation-anion pair based on a total potential solvent volume per unit cell volume of 32737.0 Å3 and a positive electron count of 2406. 3. Results and discussion 3.1. Syntheses

P P R1 ¼ jjF o j  jF c jj= jF o j. P P 2 wR2 ¼ f½ wðF o  F 2c Þ2 = ðwjF 2o jÞ2 g1=2 .

hexane molecules per molecule of the complex based a total potential solvent volume per unit cell volume of 6328.0 Å3 and a positive electron count of 605. The asymmetric unit of [CoCp2][{TpiPrMoO}2(l-S)(l-S2)] contained half a molecule, the complementary, symmetry-related half being generated by a crystallographic 2-fold axis through the l-S atom and mid-point of the S–S bond. The structure was refined using anisotropic thermal parameters for all non-hydrogen atoms. Examination of the structure with PLATON [30] showed that it

Dioxo-Mo(VI) scorpionate complexes, TpiPrMoO2X, are well known and a variety of co-ligands (X), inter alia, alcoholates [24], phenolates [25,31,32], thiolates [22,25], and thiophenolates [22,24], have been incorporated into complexes of this type. A number of additional derivatives, containing the aromatic thiolates, X = SCHPh2, SC6H4F-4, SC6H4R-2 (R = iPr, tBu, Br) and SC7H82, were prepared as part of this study. These complexes exhibited spectroscopic properties similar to those of known derivatives. IR spectra revealed two bands in the 930–890 cm1 region, assigned to the symmetric and anti-symmetric stretches of the cis-MoO2 unit, and bands due to the TpiPr and X ligands. NMR spectra were consistent with molecular Cs symmetry, with resonances readily assigned to the TpiPr and X ligands (see assignments in Experimental Section). These and related thiolate complexes react with tertiary phosphines in acetonitrile to produce acetonitrile adducts [22] that served as the immediate precursors for [TpiPrMo2V O]2(l-S)(l-S2). Attempts to prepare TpiPrMoOS(SR) complexes by reacting TpiPrMoO(SR)(NCMe) with excess propylene sulfide (or cyclohexene sulfide) in toluene under anaerobic conditions resulted in the formation of brown, diamagnetic [TpiPrMoO]2(l-S)(l-S2) (Scheme 2). The reaction is quite general and has been demonstrated for all available thiolate precursors. Selected reactions, e.g., of

Table 2 Selected distances (Å) and angles (°). Parametera

[TpiPrMoO]2(l-S)(l-S2)

[CoCp2][{TpiPrMoO}2(l-S)(l-S2)]

Mo1–O1 (Mo2–O2) Mo1–S1 (Mo2–S1) Mo1–S2 (Mo2–S3) Mo1–N11 (Mo2–N41) Mo1–N21 (Mo2–N51) Mo1–N31 (Mo2–N61) S2–S3 (S2–S20 ) N11–Mo1–O1 (N41–Mo2–O2) N11–Mo1–S1 (N41–Mo2–S1) N11–Mo1–S2 (N41–Mo2–S3) N11–Mo1–N21 (N41–Mo2–N51) N11–Mo1–N31 (N41–Mo2–N61) N21–Mo1–O1 (N51–Mo2–O2) N21–Mo1–S1 (N51–Mo2–S1) N21–Mo1–S2 (N51–Mo2–S3) N21–Mo1–N31 (N51–Mo2–N61) N31–Mo1–O1 (N61–Mo2–O2) N31–Mo1–S1 (N61–Mo2–S1) N31–Mo1–S2 (N61–Mo2–S3) Mo1–S1–Mo2(10 ) S1–Mo1–S2 (S1–Mo2–S3) Mo1–S2–S3(S2’) (Mo2–S3–S2) DMo–equatorial)b DS2, DS3c D (MoO)Mo/Mo2S3d

1.692(3), 1.678(3) 2.2800(11), 2.2816(11) 2.3340(12), 2.3330(12) 2.376(3), 2.381(4) 2.234(3), 2.218(4) 2.223(3), 2.233(4) 2.1198(15) 161.64(12), 161.27(12) 95.56(9), 95.72(8) 88.29(9), 88.54(9) 79.79(12), 80.09(12) 77.25(12), 77.15(12) 85.66(13), 85.76(13) 170.49(9), 168.99(9) 87.16(9), 85.27(9) 85.35(12), 85.42(13) 90.55(13), 89.60(13) 101.81(9), 103.68(10) 164.68(9), 164.08(10) 136.56(5) 84.40(4), 84.45(4) 116.43(5), 116.56(6) 0.1874(15), 0.1810(15) 0.2244(34), 0.0483(34) 86.41(7)

1.653(3) 2.3596(7) 2.2860(11) 2.432(4) 2.245(3) 2.273(3) 2.152(2) 160.93(13) 94.67(8) 88.16(9) 79.96(12) 76.64(12) 86.59(14) 173.44(10) 87.00(9) 83.09(13) 88.43(14) 99.42(9) 163.08(10) 127.44(7) 89.07(4) 116.82(3) 0.1976(14) 0.0963(13) 83.15(10)

a The two values listed for [TpiPrMoO]2(l-S)(l-S2) correspond to the parameters indicated, e.g., the Mo(1)–O(1) and Mo(2)–O(2) distances are 1.692(3) and 1.678(3) Å, respectively. For [CoCp2][{TpiPrMoO}2(l-S)(l-S2)], the single value for the symmetry related half-molecules is given, i.e., d(Mo(1)–O(1)) = d(Mo(10 )–O(10 ) = 1.653(3) Å. b Displacement of Mo atom from the N2S2 equatorial plane perpendicular to Mo@O. c Displacement of the S(2) and S(3) atoms from the plane defined by the Mo2(l-S) unit. d Dihedral angles between the (Mo@O)Mo unit(s) and the Mo2S3 core unit.

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TpiPrMoO(SC6H4R-4)(NCMe) (R = Me, F) with S-atom donors, monitored by 1H and/or 19F NMR spectroscopy showed the initial formation of [TpiPrMoO]2(l-S)(l-S2) and the organic disulfide; extended reaction times led to the formation of other products including, e.g., (4-FC6H4)2S. The disulfide of diphenylmethanethiol was isolated from the reaction of TpiPrMoO(SCHPh2)(NCMe) with propylene sulfide and was identified by IR and 1H NMR spectroscopy. Use of sub-stoichiometric quantities of propylene sulfide did not produce TpiPrMoOS(SR) but led instead to lower yields of the dinuclear species (and unreacted starting material). The reactions did not proceed in acetonitrile, possibly because of competitive binding of the solvent at the site required for effective SAT to Mo(IV). The use of elemental sulfur as the source of sulfur led to the formation of a complex mixture containing numerous products. Purification of [TpiPrMoO]2(l-S)(l-S2) was achieved by column chromatography under strictly anaerobic conditions, the presence of air resulting in the production of [TpiPrMoO]2(l-O)(l-S2). The complex is air-stable in the solid state for several days but required an atmosphere of dinitrogen for extended storage to prevent oxo exchange into the l-S bridge. Microanalytical and mass spectrometric data for [TpiPrMoO]2 (l-S)(l-S2) were consistent with the proposed formulation. The infrared spectrum of the complex confirmed the presence of TpiPr and oxo ligands, a single strong m(Mo@O) band at 923 cm1 being consistent with the anti disposition of the oxo groups. The 1H NMR spectrum of the complex revealed TpiPr ligands with local C1 symmetry. Thus, the five doublet resonances at d 1.06 (6H), 1.15 (6H), 1.21 (12H), 1.30 (6H), 1.32 (6H) are assigned to six pairs of inequivalent isopropyl CH3 groups, while the three septet resonances at d 3.78, 4.54, 4.63 (each 2H) are assigned to isopropyl CH protons. The higher field resonances are assigned to substituents on the rings trans to the oxo ligands [33]. A set of two doublets at d 6.13 (4H) and 6.20 (2H) and three doublets at d 7.56, 7.66, 7.68 (each 2H) are assigned to the 4- and 5-CH pyrazole ring protons. This resonance pattern is consistent with a dinuclear complex possessing C2 symmetry, the C2 axis passing through the sulfido ligand and mid-point of the disulfido bond. The diamagnetism of the complex indicates spin pairing of the formally d1 metal centers, as commonly observed for dinuclear Mo(V) complexes. The electrochemical properties of [TpiPrMoO]2(l-S)(l-S2) and [TpiPrMoO]2(l-O)(l-S2) were examined using cyclic voltammetry. Scans revealed a reversible, one-electron reduction at E = 0.56 V and 0.50 V, respectively. Additional oxidation or reduction processes were not observed in the accessible potential range. The electrochemical results indicate that the isovalent Mo(V) complexes are reduced to a stable mixed-valent Mo(IV,V) species. Chemical reduction of [TpiPrMoO]2(l-E)(l-S2) (E = O, S) with cobaltocene in a minimum volume of toluene resulted in the immediate precipitation of light brown crystals, characterized as the mixedvalent species, [CoCp2][{TpiPrMoO}2(l-E)(l-S2)] (Scheme 2). The IR spectra of the mixed-valent complexes exhibited bands 1 [34]) and due to the CoCpþ 2 cation (ca. 1415, 860 and 458 cm iPr 1 the Tp ligand (ca.m(BH) 2480 cm and m(CN) 1508 cm1). The m(Mo@O) bands at 920 cm1 (E = S) and 942 cm1 (E = O) are lower in energy than the starting materials. The mixed-valent complexes were EPR-active, exhibiting broad resonances around g = 1.99 (see Fig. 1). These signals were broader than commonly observed for Mo(V) species and they showed no obvious 95,97Mo hyperfine structure. Broadening of EPR signals has been observed with other systems involving dimeric Mo centers in close proximity [35]. Frozen-glass EPR spectra were anisotropic with g values as high as 2.172. The lack of discernable Mo hyperfine structure and the high g1 value, substantially above the value for the free electron (2.023), suggests significant localization of the unpaired electron on the bridging sulfur atoms.

Fig. 1. EPR spectra and g values of [CoCp2][{TpiPrMoO}2(l-S)(l-S2)] in 10:1 THF/ MeCN (a) at 298 K and (b) at 110 K.

3.2. Crystal and molecular structures The molecular structure of [TpiPrMoO]2(l-S)(l-S2) is shown in Fig. 2. The molecule is comprised of two distorted-octahedral oxo-Mo(V) centers bridged by l-sulfido and l-disulfido-jS:jS0 ligands; the two oxo groups adopt an anti conformation and each Mo center is further coordinated by a facial, tridentate TpiPr ligand. The molecule exhibits pseudo-C2 symmetry in the solid state, the axis running through the l-sulfido ligand and the mid-point of the l-disulfido ligand. The torsion angle of the O@Mo  Mo@O unit is 172.4(2)°. The molybdenum atoms lie ca. 0.18 Å out of the equatorial planes defined by S(1), S(2), N(21), N(31) (for Mo(1)) and S(1), S(3), N(51), N(61) (for Mo(2)). The typically short Mo@O distances of 1.692(3) and 1.678(3) Å are within the range expected for terminal oxo ligands on Mo(V). The Mo2S3 core unit is essentially planar (rms deviation = 0.0781 Å) with the disulfido ligand slightly puckered with respect to the Mo(l-S)Mo plane. Thus, atoms S(2) and S(3) are 0.224(3) Å and 0.048(3) Å either side of the Mo(l-S)Mo plane and the Mo–(l-S2)–Mo unit possesses a torsion angle of 15.13(9)°. The bridging core is characterized by Mo–(l-S) and Mo–(l-S2) distances of 2.280(1)/2.282(1) Å and 2.334(1)/ 2.333(1) Å, respectively, and Mo–(l-S)–Mo, (l-S)–Mo–(l-S2) and Mo–S–S angles of 136.56(5)°, 84.40(4)/84.45(4)° and 116.43(5)/ 116.56(6)°, respectively. The S–S distance in the l-disulfide ligand is 2.1198(15) Å, slightly longer than expected for a single bond. As expected, a lengthening of the Mo–N bonds trans to the terminal

Fig. 2. ORTEP projection of [TpiPrMoO]2(l-S)(l-S2). Ellipsoids are drawn at the 30% probability level and hydrogen atoms have been omitted for clarity. The numbering schemes for the partially labeled pyrazole rings parallel that shown for the ring containing N(11).

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oxo ligands relative to the others in the molecule is produced by the trans influence of the oxo ligand. The structure is analogous to those previously reported for [TpiPrMoO]2(l-O)(l-S2) [2] and [Tp*MoO]2(l-O)(l-S2) [15]. Crystals of [CoCp2][{TpiPrMoO}2(l-S)(l-S2)] contain discrete cations and anions as shown in Fig. 3. The structure of the anionic complex is very similar to that of [TpiPrMoO]2(l-S)(l-S2). It contains two distorted octahedral oxo-Mo centers bridged by l-sulfido and l-disulfido-jS,jS0 ligands, the coordination sphere of each Mo being completed by a facial, tridentate TpiPr ligand. The molybdenum atoms are displaced 0.198(1) Å from their respective equatorial planes (S(1), S(2), N(21) and N(31) etc). The Mo@O bond length of 1.653(3) Å is slightly shorter than those found in the isovalent analog (vide supra), whereas the S–S (2.152(2) Å) and average Mo–N bond lengths are slightly longer. The Mo–(l-S)–Mo angle is less obtuse than that in the isolvalent complex (127.4° cf 136.5°). Again, the molybdenum centers and the sulfur core atoms are non-planar, giving an Mo(1)–S(2)–S(20 )–Mo(10 ) torsion angle of 10.6(1)°. The molybdenum-oxo bonds are perpendicular to the plane of the molybdenum centers and their equatorial donors (90.00(5)°). The cations and anions are associated through two types of interactions in the crystal lattice (Fig. 4). In the first, there is a close association (tight ion-pair) between each CoCpþ 2 cation and a ldisulfido jS:jS0 bridge from one of the anions. This interaction results in Co  S distances of ca. 4.59 Å and two short C–HCp  S interactions at 3.129 and 3.162 Å. As well, the Cp rings are angled in towards the bridging disulfido unit, to form an inter-ring dihedral angle ca. 4.8°. These ion-pairs are arranged in alternating right- and left-handed helical columns around three-fold screw axes along the c-axis, as shown in Fig. 4a. There is a second, weaker interaction where the peripheral l-sulfido ligand of every tight ion-pair projects between two CoCpþ 2 ions of an adjacent column; here, the Co  l-S and C–HCp  l-S distances are 6.260 and >3.912 Å, respectively (Fig. 4b). Each column of CoCpþ 2 ions is characterized by alternating interactions of the two types described above, the anions acting as linkers from any one column of CoCpþ 2 ions to three neighboring columns; a view from the top of one such column is given in Fig. 5. In the complete structure, the helical columns along the c axis are linked to form a honeycomblike, supramolecular structure, having a hexagonal projection in

the ab lattice plane (Fig. 6). This arrangement results in the formation of large channels, surrounded by 6 of the pillars. The TpiPr ligands of the anions face in towards these channels making them hydrophobic in nature. The channels contain tetrahydrofuran solvent molecules (3.33 per cation-anion pair) that are highly disordered and resistant to modeling. A preliminary but unpublishable structure for the oxo analog [CoCp2][{TpiPrMoO}2(l-O)(l-S2)] revealed the absence of a supramolecular structure in that compound.

Fig. 3. ORTEP projection of a cation–anion pair in [CoCp2][{TpiPrMoO}2(l-S)(l-S2)]. Ellipsoids are draw at the 30% probability level and hydrogen atoms have been omitted for clarity. The numbering schemes for the pyrazole rings parallel that shown for the ring containing N(11) in Fig. 2.

Fig. 5. View down one of the cation–anion columns in the structure of [CoCp2][{TpiPrMoO}2(l-S)(l-S2)]. The CoCpþ 2 cations are shown in light blue, the closely associated anions in dark blue, and anions from neighboring columns in red, purple and green.

Fig. 4. Two side-on views of the columns of CoCpþ 2 cations and associated anions in the structure of [CoCp2][{TpiPrMoO}2(l-S)(l-S2)]. (a) Tight ion-pairs involving Mo2 ðl-S2 Þ    CoCpþ 2 contacts. (b) As per (a) but also including anions most-closely associated with neighboring columns. The red, purple and green anions are associated with three different columns disposed at 120° around the column shown. The l-S ligands of the red, purple and green anions project into the region between the CoCpþ 2 cations of the column shown.

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Acknowledgements We thank Ms. Sally Duck, Monash University, for the ESI-MS data and Assoc. Profs. Brendan F. Abrahams and Jonathan M. White for helpful discussions relating to the crystal structures. We gratefully acknowledge the financial support of the Australian Research Council. Appendix A CCDC 749882 and 749883 contain the supplementary crystal structure data for [TpiPrMoO]2(l-S)(l-S2)3(C6H14) and [CoCp2][{TpiPrMoO}2(l-S)(l-S2)]3.33(C4H8O), respectively. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. References

Fig. 6. Supramolecular structure of [CoCp2][{TpiPrMoO}2(l-S)(l-S2)], showing the hexagonal array of interconnected columns of ion-pairs (as viewed down c axis). The Mo and Co atoms are highlighted as purple and green spheres, respectively.

4. Conclusions The reactions of oxo-Mo(IV) phenolate complexes, TpiPrMoO(OAr)L (L = OPR3, NCMe) with the sulfur atom transfer reagent propylene sulfide result in the formation of oxosulfido-Mo(VI) complexes, TpiPrMoOS(OAr) [1,2]. In stark contrast, thiolate complexes, TpiPrMoO(SR)(NCMe), react with propylene sulfide to form the dinuclear Mo(V) complex, [TpiPrMoO]2(l-S)(l-S2). The formation of this compound could not be prevented, even with the use of sub-stoichiometric amounts of propylene sulfide or sterically hindered thiolate co-ligands. The mechanism of the reaction is unclear but may involve initial formation of oxosulfido-Mo(VI) complexes, TpiPrMoOS(SR). These may decompose by a number of routes, e.g., thiolate ligand dissociation or dimerization (as occurs in some phenolate derivatives [1,2]), followed by thiol oxidation and disulfide formation; the net reaction is shown in Scheme 2. Thus, the redox interplay of the product and the reagent necessary for its very formation appear to conspire against the isolation of mononuclear TpiPrMoOS(SR) complexes. This observation suggests that the combination of oxo, sulfido and thiolate ligands at Mo(VI) may be difficult to achieve synthetically. This conclusion can also be drawn from the work of others [14] and the fact that there is no precedent for synthetic complexes containing an [MoOS]2+ core and a thiolate or dithiolene ligand. The protein environment and structural constraints at the active sites of Mo hydroxylases are likely to be essential for the stabilization of the [(MPT)MoVIOS(OH)] catalytic center and the elimination of undesirable redox reactions involving the sulfur-based ligands. The Mo(V) complex, along with its l-oxo analog, are amenable to reduction to the mixed-valent species, [CoCp2][{TpiPrMoO}2(lE)(l-S2)] (E = S or O, respectively), a new class of mixed-valent Mo complex. The l-sulfido complex exhibits supramolecular interactions forming a honeycomb structure with large solvent filled voids. A preliminary crystal structure of the l-oxo analog shows that it crystallizes in a lattice devoid of such supramolecular interactions. It appears that polarisable sulfur ligands on opposite sides of the Mo2(l-S)(l-S2) core unit are necessary to facilitate the supramolecular interactions observed in [CoCp2][{TpiPrMoO}2 (l-S)(l-S2)].

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