Organometallics 2009, 28, 749–754
749
Determination of S-H Bond Strengths in Dimolybdenum Tetrasulfide Complexes Aaron M. Appel,† Suh-Jane Lee,† James A. Franz,*,† Daniel L. DuBois,† M. Rakowski DuBois,† and Brendan Twamley‡ Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, and the Department of Chemistry, UniVersity of Idaho, Moscow, Idaho 83844-2343 ReceiVed September 8, 2008
Homolytic solution bond dissociation free energies (SBDFE) for S-H bonds have been determined for soluble dimolybdenum tetrasulfide complexes through thermochemical cycles using electrochemical potentials and pKa values in acetonitrile. In spite of the importance and extensive use of metal sulfide catalysts, these S-H bond strengths are among the first experimentally determined values for metal sulfide systems. For [Cp*Mo(µ-S)(µ-SMe)2(µ-SH)MoCp*]+ (S4Me2H+), [Cp*Mo(µ-S)(µ-SMe)(µ-SH)2MoCp*]+ (S4MeH2+), and [Cp*Mo(µ-S)(µ-SH)3MoCp*]+ (S4H3+), the pKa values were determined to be 5.6 ( 0.4, 5.3 ( 0.3, and 4.9 ( 0.3, respectively. The E1/2 values for S4Me2•+/o, S4MeH•+/o, and S4H2•+/o were measured to be -0.02 ( 0.02, +0.04 ( 0.05, and +0.07 ( 0.07 V vs FeCp2+/o, respectively. Using these experimental values, the homolytic S-H SBDFE for S4Me2H+ to S4Me2•+, S4MeH2+ to S4MeH•+, and S4H3+ to S4H2•+ were determined to be 60.8 ( 1.0, 61.8 ( 1.6, and 61.9 ( 2.0 kcal/mol, respectively. These SBDFE values can be used to estimate gas phase bond dissociation enthalpies of 65.6, 66.6, and 66.7 kcal/mol, respectively. Solid state structures are presented for S4MeH and S4H2. Introduction While metal sulfides are extensively used in hydrodesulfurization catalysts1-3 and in enzymes,3-5 few experimental MS-H bond strengths have been reported.6 Soluble metal sulfide model complexes based on a Mo2S4 core7-10 have been studied for their ability to form and cleave bonds at the bridging sulfurs.11-23 The sulfur based reactivity that is observed for these metal sulfides24 can result in catalysts that are based on * Corresponding author. E-mail:
[email protected]. † Pacific Northwest National Laboratory. ‡ University of Idaho. (1) Grange, P.; Vanhaeren, X. Catal. Today 1997, 36, 375–391. (2) Bianchini, C.; Meli, A. Acc. Chem. Res. 1998, 31, 109–116. (3) Donahue, J. P. Chem. ReV. 2006, 106, 4747–4783. (4) Hille, R. Chem. ReV. 1996, 96, 2757–2816. (5) Brondino, C. D.; Rivas, M. G.; Romao, M. J.; Moura, J. J. G.; Moura, I. Acc. Chem. Res. 2006, 39, 788–796. (6) Appel, A. M.; DuBois, D. L.; Rakowski DuBois, M. J. Am. Chem. Soc. 2005, 127, 12717–12726. (7) King, R. B. J. Am. Chem. Soc. 1963, 85, 1587–1590. (8) Dahl, L. F.; Connelly, N. G. J. Am. Chem. Soc. 1970, 92, 7470– 7472. (9) Wachter, J. Angew. Chem., Int. Ed. Engl. 1989, 28, 1613–1626. (10) Rakowski DuBois, M. Chem. ReV. 1989, 89, 1–9. (11) Rakowski DuBois, M.; Haltiwanger, R. C.; Miller, D. J.; Glatzmaier, G. J. Am. Chem. Soc. 1979, 101, 5245–5252. (12) Rakowski DuBois, M.; Vanderveer, M. C.; DuBois, D. L.; Haltiwanger, R. C.; Miller, W. K. J. Am. Chem. Soc. 1980, 102, 7456– 7461. (13) Kubas, G. J.; Ryan, R. R.; Kubatmartin, K. A. J. Am. Chem. Soc. 1989, 111, 7823–7832. (14) Bernatis, P.; Laurie, J. C. V.; Rakowski DuBois, M. Organometallics 1990, 9, 1607–1617. (15) Birnbaum, J.; Godziela, G.; Maciejewski, M.; Tonker, T. L.; Haltiwanger, R. C.; Rakowski DuBois, M. Organometallics 1990, 9, 394– 401. (16) Mansour, M. A.; Curtis, M. D.; Kampf, J. W. Organometallics 1997, 16, 275–284. (17) Newell, R.; Ohman, C.; Rakowski DuBois, M. Organometallics 2005, 24, 4406–4415. (18) Franz, J. A.; Birnbaum, J. C.; Kolwaite, D. S.; Linehan, J. C.; Camaioni, D. M.; Dupuis, M. J. Am. Chem. Soc. 2004, 126, 6680–6691.
inexpensive metals and are not susceptible to typical catalyst poisons such as CO.6 An understanding of bond strengths is important for a wide range of reactions, including hydrogenation,25-29 chain transfer polymerization,30-34 and initiation of other radical reactions.35-37 While heterogeneous and enzymatic reactions are ubiquitous, thermodynamic studies of bond formation and cleavage reactions in these systems is quite difficult. The use of small molecule model complexes as surrogates for these more complex systems9,10 allows for the determination of analogous bond strengths. (19) Rakowski DuBois, M.; Vasquez, L. D.; Ciancanelli, R. F.; Noll, B. C. Organometallics 2000, 19, 3507–3515. (20) Farmer, M. M.; Haltiwanger, R. C.; Kvietok, F.; Rakowski DuBois, M. Organometallics 1991, 10, 4066–4070. (21) Brunner, H.; Meier, W.; Wachter, J.; Weber, P.; Ziegler, M. L.; Enemark, J. H.; Young, C. G. J. Organomet. Chem. 1986, 309, 313–318. (22) Casewit, C. J.; Coons, D. E.; Wright, L. L.; Miller, W. K.; Rakowski DuBois, M. Organometallics 1986, 5, 951–955. (23) Brunner, H.; Kauermann, H.; Meier, W.; Wachter, J. J. Organomet. Chem. 1984, 263, 183–192. (24) Kubas, G. J. Chem. ReV. 2007, 107, 4152–4205. (25) Bullock, R. M.; Samsel, E. G. J. Am. Chem. Soc. 1990, 112, 6886– 6898. (26) Ru¨chardt, C.; Gerst, M.; Ebenhoch, J. Angew. Chem., Int. Ed. Engl. 1997, 36, 1406–1430. (27) Sweany, R. L.; Halpern, J. J. Am. Chem. Soc. 1977, 99, 8335– 8337. (28) Halpern, J. Pure Appl. Chem. 1986, 58, 575–584. (29) Eisenberg, D. C.; Norton, J. R Isr. J. Chem 1991, 31, 55–66. (30) Gridnev, A. A.; Ittel, S. D. Chem. ReV. 2001, 101, 3611–3660. (31) Tang, L.; Norton, J. R. Macromolecules 2006, 39, 8236–8240. (32) Tang, L.; Norton, J. R. Macromolecules 2006, 39, 8229–8235. (33) Choi, J.; Tang, L.; Norton, J. R. J. Am. Chem. Soc. 2007, 129, 234–240. (34) Tsarevsky, N. V.; Matyjaszewski, K. Chem. ReV. 2007, 107, 2270– 2299. (35) Yet, L. Tetrahedron 1999, 55, 9349. (36) Jasperse, C. P.; Curran, D. P.; Fevig, T. L. Chem. ReV. 1991, 91, 1237–1286. (37) Smith, D. M.; Pulling, M. E.; Norton, J. R. J. Am. Chem. Soc. 2007, 129, 770–771.
10.1021/om800875n CCC: $40.75 2009 American Chemical Society Publication on Web 01/12/2009
750 Organometallics, Vol. 28, No. 3, 2009
Appel et al.
Chart 1. Structural Representations and Abbreviations of Cp*2Mo2S4 Complexes in this Work (Cp* ) η5-C5Me5)
To our knowledge, only one experimentally derived homolytic S-H bond strength has been reported for a homogeneous Mo2S4 system.6 Homolytic bond strengths can be determined from pKa and E1/2 values using thermochemical cycles like those of Breslow and Balasubramanian38 and later Nicholas and Ar-
Figure 1. Thermal displacement (30%) diagram of the major disordered component (29% S1a S2a; 20% C11a) of S4MeH. Hydrogen atoms omitted for clarity. Note that the structure has the general appearance of S4Me2, but the total occupancy of the methanethiolate carbon atoms is only 50% of that for S4Me2, consistent with S4MeH with disorder between the methanethiolate and hydrosulfide.
nold,39 as described by Bordwell for a variety of organic molecules40 and Tilset and Parker for metal hydrides.41 This approach allows for the determination of bond strengths that are otherwise difficult to measure. By applying this technique to Mo2S4 complexes, homolytic S-H solution bond dissociation free energies (SBDFEs) have been determined. These experimental values are among the first determined S-H bond energies in metal sulfides and can serve as the basis for the
Figure 2. Thermal displacement (30%) diagram of the major disordered component (50% C1a-C5a′; 50% S2a) of S4H2. Hydrogen atoms other than S-H are omitted for clarity.
S-H Bond Strengths in Cp*2Mo2S4 complexes
Organometallics, Vol. 28, No. 3, 2009 751
Table 1. Selected Bond Lengths and Angles for S4MeH distances, Å
angles, deg
Table 2. Selected Bond Lengths and Angles for S4H2 distances, Å
angles, deg
bond
value
angle
value
bond
value
angle
value
Mo(1)-Mo(1A) Mo(1)-S(1A) Mo(1)-S(1AA) Mo(1)-S(2A) Mo(1)-S(2AA) S(1A)-C(11A) Mo-C (average)
2.5711(2) 2.496(3) 2.462(4) 2.393(6) 2.374(5) 1.802(9) 2.342
Mo(1)-S(1A)-Mo(1A) Mo(1)-S(2A)-Mo(1A) S(1A)-Mo(1)-S(2A) S(1A)-Mo(1)-S(2AA) C(11A)-S(1A)-Mo(1) C(11A)-S(1A)-Mo(1A)
62.48(9) 65.28(10) 75.80(13) 71.06(14) 111.9(7) 112.8(7)
Mo(1)-Mo(1A) Mo(1)-S(1) Mo(1)-S(1A) Mo(1)-S(2A) Mo(1)-S(2AA) Mo-C (average)
2.5667(8) 2.3408(19) 2.343(2) 2.473(11) 2.446(13) 2.342
Mo(1)-S(1)-Mo(1A) Mo(1)-S(2A)-Mo(1A) S(1)-Mo(1)-S(2A) S(1A)-Mo(1)-S(2A)
66.46(6) 62.9(3) 67.2(3) 78.8(3)
validation of electronic energy calculations in these complex systems. The bond energies reported throughout this work are free energies, unless otherwise stated, as all of the determined thermodynamic values are derived from electrochemical potentials and pKa values, both of which are free energy measurements. Additionally, prediction of solution equilibria requires knowledge of free energies and not just bond enthalpies. For comparison to previously reported bond energies, SBDFE values can be converted to gas phase bond dissociation enthalpies (BDEs) by addition of 4.8 kcal/mol.42
Results and Discussion Syntheses and Structures. A new synthetic route has been utilized to prepare S4Me2 in higher yield than previously reported.21 Reaction of S4 with methyllithium results in the formation of the methylated anion, S4Me-. Subsequent addition of a methyl cation by reaction with methyl iodide gives the neutral dimethylated complex, S4Me2. Spectroscopic data for the product of this synthetic route match the reported data.21 S4MeH and S4H2 were synthesized as reported in the literature,12,43 and single crystal X-ray structures were collected for each. For S4MeH, the methanethiolate and hydrosulfide ligands are disordered in the crystal structure, thereby giving a solid state structure that appears to be very similar to that of S4Me2.21 However, the methanethiolate carbon atoms of S4MeH have only a 50% occupancy, and the 1H NMR spectrum43 shows Cp* methyl, methanethiolate, and hydrosulfide resonances in a 30: 3:1 ratio with chemical shifts that are similar to but different from S4Me2 and S4H2. The sulfides and methanethiolates are disordered in the structure, giving an average S-C bond length of 1.75 Å for the methanethiolates and a Mo-Mo distance of 2.57 Å. The structure is shown in Figure 1, with selected bond lengths and angles in Table 1. The solid state structure of S4H2 was previously reported,44 but did not include the complete data. For this reason, the structure was recollected, and is similar to those for S4MeH and S4Me2. The sulfides and hydrosulfides are disordered into multiple positions, giving an average Mo-S distance of 2.34 Å for the sulfides and 2.47 Å for the hydrosulfides. The Mo-Mo distance is 2.57 Å, typical for structures of related complexes20,21,43 and the structure of S4MeH, above. The structure is shown in Figure 2, with selected bond lengths and angles in Table 2. (38) Breslow, R.; Balasubramanian, K. J. Am. Chem. Soc. 1969, 91, 5182–5183. (39) Nicholas, A. M. D.; Arnold, D. R Can. J. Chem. 1982, 60, 2165– 2179. (40) Bordwell, F. G.; Cheng, J. P.; Harrelson, J. A. J. Am. Chem. Soc. 1988, 110, 1229–1231. (41) (a) Tilset, M.; Parker, V. D J. Am. Chem. Soc. 1989, 111, 6711– 6717. (b) As modified in: J. Am. Chem. Soc., 1990, 112, 2843. (42) Wayner, D. D. M.; Parker, V. D. Acc. Chem. Res. 1993, 26, 287– 294. (43) Appel, A. M.; Lee, S.-J.; Franz, J. A.; DuBois, D. L.; Rakowski DuBois, M.; Birnbaum, J. C.; Twamley, B. J. Am. Chem. Soc. 2008, 130, 8940–8951. (44) Shin, J. H.; Parkin, G. Polyhedron 1994, 13, 1489–1493.
pKa Measurements in Acetonitrile. Protonation of S4Me2, S4MeH, and S4H2 gives S4Me2H+, S4MeH2+, and S4H3+, respectively (eq 1). For aniline derivatives as well as the Mo2S4 complexes, the acid and base forms exchange rapidly on the NMR time scale, resulting in averaged peaks in CD3CN. Given that the chemical shifts are known for the protonated and deprotonated forms of the anilines used as well as the Mo2S4 complexes, the weighted average of the chemical shifts for each species in the equilibrium mixtures gives the ratios of the acid and base form of each. These ratios were used to determine the equilibrium constants for eq 1a, which can be combined with the known pKa values of the reference acids (BH+) to calculate the pKa of the analyte (e.g., S4Me2H+), as in eq 2:
S4Me2 + BH+ h S4Me2H+ + B
(1a)
S4MeH + BH+ h S4MeH2+ + B
(1b)
S4H2 + BH+ h S4H3+ + B
(1c)
pKa_analyte ) pKa_ref+log(Keq)
(2)
where BH+ is the acid form of the reference, B is the base form of the reference, and Keq is the equilibrium constant for eq 1. Using this method, the relative pKa values of reference acids reported in the self-consistent spectrophotometric basicity scale by Kaljurand et al.45 have been found to agree within 0.2 pKa units of the reported values for 2,6-dichloroanilinium, 2,5dichloroanilinium, p-(trifluoromethyl)anilinium, p-bromoanilinium, anilinium, and p-anisidinium. Due to the availability of this large, self-consistent scale in acetonitrile, all of the pKa values for reference compounds are taken from this source. Additionally, the pKa value of p-cyanoanilinium was determined against 2,5-dichloroanilinium and p-(trifluoromethyl)anilinium and found to be 7.0 ( 0.2, corrected from the previously reported46 value of 7.6. Using 1H NMR, equilibria between S4Me2, S4MeH, or S4H2 and 2,6-dichloroanilinium (pKa ) 5.06) or 2,5-dichloroanilinium (pKa ) 6.21) were measured in CD3CN, resulting in pKa values of 5.6 ( 0.4, 5.3 ( 0.3, and 4.9 ( 0.3 for S4Me2H+, S4MeH2+, and S4H3+, respectively. The pKa values shift in the expected direction: replacing methanethiolates with hydrosulfides results in complexes that are more acidic, as the hydrogens are less electron donating than the methyl groups. In the case of S4MeH2+ and S4H3+, the pKa equilibria were run under a hydrogen atmosphere to prevent loss of H2 (the conversion of S4H3+ to S4H+ by loss of H2 has been previously reported in the synthesis of S4 from S4H2,43 see Supporting Information for additional details). Electrochemical Measurements. Cyclic voltammograms (CV’s) of S4Me2 in 0.3 M NEt4BF4 in acetonitrile show both a (45) Kaljurand, I.; Kutt, A.; Soovali, L.; Rodima, T.; Maemets, V.; Leito, I.; Koppel, I. A. J. Org. Chem. 2005, 70, 1019–1028. (46) Edidin, R. T.; Sullivan, J. M.; Norton, J. R. J. Am. Chem. Soc. 1987, 109, 3945–3953.
752 Organometallics, Vol. 28, No. 3, 2009
Appel et al. Scheme 1. Thermochemical Data for S4Me2, Showing the Relationships between pKa, E1/2, and SBDFE Valuesa
Figure 3. Cyclic voltammograms of S4Me2 (blue trace) and S4MeH (red trace) in 0.3 M NEt4BF4 in acetonitrile at a scan rate of 50 mV/s.
reversible reduction at -2.04 V and a reversible oxidation at -0.02 V (all potentials have been referenced to the ferrocenium/ ferrocene couple). While CV’s of S4MeH show a similar reversible reduction at -1.96 V, the oxidation wave at +0.04 V become completely irreversible at scan rates below 0.5 V/s, as shown in Figure 3. At scan rates of 5 V/s, 50 V/s, and 100 V/s, the return wave becomes increasingly reversible. Similarly, CV’s of S4H2 again show a reversible reduction, shifted to -1.87 V, and an oxidation that is irreversible at low scan rates. At scan rates of 5 to 200 V/s, the potential for this oxidation was determined to be +0.07 V. Table 3 shows the positive shift in potentials in moving from S4Me2 to S4MeH to S4H2, consistent with replacing the more electron donating methyl groups with hydrogen atoms. CV’s for the protonated species, such as S4Me2H+, showed reduction waves that were electrochemically and chemically irreversible. The potentials for these reductions were not pursued due to the complications that arise when fast following reactions are neither well understood nor clean. Determination of Solution Bond Dissociation Free Energies. While solution bond dissociation free energies (SBDFEs) were not directly measured, they can be calculated42,47 from the experimentally determined pKa values and electrochemical potentials. One example of this is shown in eqs 3–7 for S4Me2H+. The pKa value for S4Me2H+ to S4Me2 (eq 3) and the potential versus FeCp2+/o for the S4Me2+/o couples (eq 4), along with the free energy for reducing a proton in acetonitrile (eq 5), add up to the homolytic S-H cleavage reaction in eq 6. Using the free energies for eqs 3–5, the SBDFE for eq 6 can be calculated using eq 7, where free energies are in kcal/mol and E1/2 is in V vs FeCp2+/o.
S4Me2H+ h S4Me2 + H+ S4Me2 h S4Me2•+ + eH+ + e- h H• S4Me2H+ h S4Me2•+ + H•
pKa
(3)
-E1/2
(4)
E1/2 homolytic SBDFE
SBDFE ) (23.06E1/2) + (1.37pKa) + 53.6
(5) (6) (7)
This results in SBDFEs that are determined through thermochemical cycles using experimental E1/2 and pKa values. The thermochemical cycles for S4Me2 are shown in Scheme 1, and the homolytic SBDFE values for S4Me2H+, S4MeH2+, and S4H3+ are shown in Table 4. Given that gas phase bond (47) Ellis, W. W.; Raebiger, J. W.; Curtis, C. J.; Bruno, J. W.; DuBois, D. L. J. Am. Chem. Soc. 2004, 126, 2738–2743. See supporting information.
a Bold values are directly measured experimental values (pKa and E1/2), whereas values in italics are determined using the measured values and thermodynamic cycles.
Table 3. Cyclic Voltammetry Data for the Mo2S4 Complexes in Acetonitrile with 0.3 M NEt4BF4 couple
E1/2 or Epa (V vs FeCp2+/o)
∆E p or Ep-Ep/2a(mV)
Scan Rate(V/s)
S4Me2•+/o S4MeH•+/o S4H2•+/o S4Me2o/•S4MeHo/•S4H2o/•-
-0.02 0.04 0.07 -2.04 -1.96 -1.87
67 122 194 67 62 63
0.5 50 200 0.5 0.5 0.5
Table 4. Experimental pKa and S-H homolytic SBDFE Values in Acetonitrile, and Estimated Gas Phase BDEs for S4MenH3-n+
S4Me2H+ S4MeH2+ S4H3+
pKa in acetonitrile
acetonitrile SBDFE (kcal/mol)
estimated gas phase BDE (SBDFE + 4.8 kcal/mol)42
5.6 (0.4 5.3 ( 0.3 4.9 ( 0.3
60.8 ( 1.0 61.8 ( 1.6 61.9 ( 2.0
65.6 66.6 66.7
dissociation enthalpies (BDEs) are expected to exceed acetonitrile SBDFE values by 4.8 kcal/mol,42 the corresponding BDE estimates are also shown in Table 4. Previous work using a related complex, CpMo(µ-S)2(µS2CH2)MoCp, resulted in similar thermochemical data, as shown in Scheme 2.6 Note, however, that this data has been corrected due to the change in the pKa of the reference base used in that study, p-cyanoanilinium (see pKa studies above). Correcting the pKa of p-cyanoanilinium from 7.6 to 7.0 decreases the determined pKa value for CpMo(µ-SH)(µ-S)(µ-S2CH2)MoCp+ to 6.5 ( 0.3. This in turn results in a lower value for the homolytic SBDFE, 49.4 ( 1.3 kcal/mol, for CpMo(µ-SH)(µ-S)(µS2CH2)MoCp to CpMo(µ-S)2(µ-S2CH2)MoCp. This SBDFE is considerably lower than those observed for the S4MenH3-n+ complexes in this study. However, the previous values are for the neutral species where an open-shell hydrogen atom donor forms a closed-shell product, whereas the current study involves the cationic S4MenH3-n+ species (closed-shell), and the S4MenH2-n•+ products (open-shell). The pKa values between the two studies are more similar than the SBDFE values, primarily because the species that are deprotonated and protonated have similar structures, charges, and oxidation states.
S-H Bond Strengths in Cp*2Mo2S4 complexes Scheme 2. Corrected Thermochemical Data from the Literature6 for CpMo(µ-S)2(µ-S2CH2)MoCp, Showing the Relationships between pKa, E1/2, and SBDFE Valuesa
a Bold values are directly measured experimental values (pK and a E1/2), whereas values in italics are determined using the measured values and thermodynamic cycles. These data have been corrected from the literature values6 due to a change in the pKa value of the reference acid.
The corresponding S-H BDE values for all of the Mo2S4 complexes are substantially lower than that of H2S (91.2)48 and organic thiols, such as thiophenol (79.1).49 The S-H SBDFE values are in the same range as many M-H bond energies (50-68 kcal/mol)42,50-52 as well as C-H bond energies in metal-formyl complexes (49-52 kcal/mol).53 In the case of the metal-formyl complexes, this decrease in C-H bond energies relative to aldehydes (83-87 kcal/mol)54,55 is similar in many ways to the decrease in S-H bond energies in Mo2S4 complexes. The SBDFE values for the current S4MenH3-n+ species as well as the previously reported Mo2S4 complex do show a significant decrease in S-H bond strength due to the sulfur coordination to the metal centers.
Summary and Conclusions The molybdenum sulfide complexes S4Me2, S4MeH, and S4H2 were synthesized, and their S-H homolytic SBDFEs were determined using a combination of pKa and E1/2 values, giving very similar SBDFE values for each complex. These are among the first experimentally determined S-H SBDFE values for metal sulfide complexes, and may give insight into the thermochemistry of more complex systems. The corresponding gas phase homolytic BDE values are significantly lower than those for H2S or organic thiols, showing the substantial effect that metal coordination has upon the S-H bond strengths. This decrease in bond energies is similar to those previously observed in M-H bonds as well as the C-H bonds of metal-formyl complexes. More extensive thermochemical studies using S4MeH and S4H2 are in progress, and the results of these studies will be used as a basis for computational efforts. (48) Nicovich, J. M.; Kreutter, K. D.; Van Dijk, C. A.; Wine, P. H. J. Phys. Chem. 1992, 96, 2518–2528. (49) Bordwell, F. G.; Zhang, X.-M.; Satish, A. V.; Cheng, J. P. J. Am. Chem. Soc. 1994, 116, 6605–6610. (50) Skagestad, V.; Tilset, M. J. Am. Chem. Soc. 1993, 115, 5077–5083. (51) Tilset, M. J. Am. Chem. Soc. 1992, 114, 2740–2741. (52) Ciancanelli, R.; Noll, B. C.; DuBois, D. L.; DuBois, M. R. J. Am. Chem. Soc. 2002, 124, 2984–2992. (53) Ellis, W. W.; Miedaner, A.; Curtis, C. J.; Gibson, D. H.; DuBois, D. L. J. Am. Chem. Soc. 2002, 124, 1926–1932. (54) McMillen, D. F.; Golden, D. M. Annu. ReV. Phys. Chem. 1982, 33, 493–532. (55) Niiranen, J. T.; Gutman, D.; Krasnoperov, L. N. J. Phys. Chem. 1992, 96, 5881–5886.
Organometallics, Vol. 28, No. 3, 2009 753
Experimental Procedures Instrumentation. 1H NMR spectra were acquired using a Varian Inova 500 or VXR-300 spectrometer, and the chemical shifts were referenced to the residual solvent peak (1.94 ppm for acetonitriled3). Electrochemical data were collected using a CH Instruments model 660C computer-aided three-electrode potentiostat in acetonitrile with 0.3 M tetraethylammonium tetrafluoroborate. For cyclic voltammetry the working electrode was a glassy carbon disk, the counter electrode was a glassy carbon rod, and a silver chloride coated silver wire was used as a pseudoreference electrode and was separated from the main compartment by a Vycor disk (1/8 in. diameter) obtained from Bioanalytical Systems, Inc. Ferrocene or decamethylferrocene was used as an internal reference with all potentials reported versus the ferrocenium/ferrocene couple. Materials. Reagents were purchased commercially and used without further purification unless otherwise specified. All reactions, syntheses, and manipulations of Mo2S4 complexes were carried out under nitrogen using standard Schlenk techniques or in a glovebox. Hydrogen was dried using a 1 cm diameter, 30 cm long column of drierite. Acetonitrile was dried by an activated alumina column in an Innovative Technology, inc, PureSolv system. CD3CN was degassed, dried over activated sieves, and stored in a glovebox. S4H2 and S4MeH were synthesized as previously described.12,43,56 Protonation of 2,6-Dichloroaniline. Trifluoromethane sulfonic acid (1.0 mL, 0.011 mol) was added to 2,6-dichloroaniline (1.24 g, 0.0077 mol) in 100 mL of diethyl ether. The resulting white precipitate was collected by filtration, washed with 80 mL diethyl ether, and dried under vacuum, yielding 1.8 g (76%). The solid was stored in a glovebox and used thereafter for the pKa studies below. Equilibrium Measurements. All measurements were made at 22 ( 3 °C. The pKa values were determined by NMR and were measured for three or more independent samples. The electrochemical potentials are the average of the measured E1/2 values for reversible couples over scan rates of 0.05 to 50 V/s where applicable. The reported errors in the acid/base equilibria included two standard deviations for the reproducibility plus an additional 0.2 pKa units to allow for uncertainty in the pKa values of the reference bases. Similarly, the reported errors in electrochemical potentials are two standard deviations for the reproducibility plus an additional 10 mV to account for any errors in the potentials of the references. pKa Determinations. The pKa value for 2,6-dichloroanilinium and 2,5-dichloroanilinium were taken from the self-consistent scale published by Kaljurand, et al.45 The pKa values for the Mo2S4 complexes were determined against known bases using 1H NMR at analyte and reference concentrations 2σ(I)] a wR2 [I > 2σ(I)] ∆F peak/hole (e Å-3) a
S4MeH
S4H2
C21H34Mo2S4 606.60 monoclinic, P2(1)/n 8.1814(3) 14.4649(5) 10.7065(4) 90 108.809(1) 90 1199.38(8) 2 90(2) 0.71073 1.680 1.399 616 0.32 × 0.17 × 0.08 2.45 to 27.50 -10 e h e 10, -18 e k e 18, -13 e l e 13 17195 2763 [R(int) ) 0.0226] 2763/3/153 1.092 0.0222 0.0536 0.781 and -0.308
C20H32Mo2S4 592.58 triclinic, P1j 8.1827(8) 8.4976(8) 10.5613(10) 102.598(2) 102.636(2) 116.3370(10) 599.04(10) 1 296(2) 0.71073 1.643 1.399 300 0.18 × 0.15 × 0.07 2.12 to 25.25 -9 e h e 9, -10 e k e 10, -12 e l e 12 8969 2179 [R(int) ) 0.0321] 2179/1/115 0.970 0.0423 0.1051 0.923 and -0.516
R1 ) Σ|Fo| - |Fc|/Σ|Fo|; wR2 ) {Σ[w(Fo2 - Fc2)2]/ Σ [w(Fo2)2]}1/2.
Synthesis of Cp*Mo(µ-S)2(µ-SMe)2MoCp*, S4Me2. A new synthetic route to S4Me2 is described below, and the spectroscopic data matches the previously reported data.21 Methyl lithium (0.10 mL of 1.6 M in diethyl ether, 0.16 mmol) was added to a solution of S4 (0.050 g, 0.085 mmol) in 20 mL toluene at 0 °C, changing the blue solution to purple. After stirring for 30 min methyl iodide was added (10 µL, 0.16 mmol), and the resulting red solution was stirred for 1.5 h. The solvent was removed under vacuum, and the solid was purified by column chromatography with benzene. Yield 0.048 g, 91%. 1H NMR (CD3CN, 500 MHz): δ 2.23 (s, 11.4 H, Cp*, isomer A), 2.21 (s, 18.6 H, Cp*, isomer B), 0.997 (s, 3.7 H, SMe, isomer B), 0.87 (s, 2.3 H, SMe, isomer A). Single Crystal X-ray Diffraction. Crystals were removed from the flask and covered with a layer of hydrocarbon oil. A suitable crystal was selected, mounted on a glass fiber, and placed in the low-temperature nitrogen stream.57 Data for compound S4MeH was collected at ca. 90(2) K and for S4H2 at 296(2) K using a Bruker/ Siemens SMART APEX instrument (Mo KR radiation, λ ) 0.71073 Å) equipped with a Cryocool NeverIce low temperature device. At lower temperatures for S4H2 (T < 273 K) the sample disintegrated. Data were measured using omega scans of 0.3 ° per frame for 20 s (5 s for S4H2), and a full sphere of data was collected. A total of 2400 frames were collected with a final resolution of 0.77 Å for S4MeH and 0.83 Å for S4H2. Cell parameters were retrieved using SMART58 software and refined using SAINTPlus59 on all observed reflections. Data reduction and correction for Lp and decay were performed using the SAINTPlus59 software. Absorption corrections were applied using SADABS.60 Both structures were solved by direct methods and refined by the least-squares method on F2 using the SHELXTL61 program package. The structure of S4MeH was refined in the space group P2(1)/n (No. 14) by analysis (57) Hope, H. Prog. Inorg. Chem. 1994, 41, 1–19. (58) SMART: V. 5.632, Bruker AXS: Madison, WI, 2005. (59) SAINTPlus: V. 7.23a, Data Reduction and Correction Program, Bruker AXS: Madison, WI, 2004. (60) SADABS: V.2004/1, an empirical absorption correction program, Bruker AXS Inc.: Madison, WI, 2004. (61) Sheldrick, G. M. SHELXTL: V. 6.14, Structure Determination Software Suite; Bruker AXS Inc.: Madison, WI, 2004.
of systematic absences. The central S/Me/H groups were disordered in a continuum in the center of the molecule. The S atoms were modeled in 6 discrete sites with refined occupancies: A, 29%; B, 26%; C, 13%; D, 22%; E, 6%; F, 4%. The terminal methyl group was localized and refined in two sites: C11a, 20%; C11b, 30%. These disordered atoms were held isotropic. The terminal hydrosulfide could not be located but was included in the formula. All other non-hydrogen atoms were refined anisotropically. The structure of S4H2 was solved in the space group P1j (No. 2) by analysis of systematic absences. The structure displayed rotational disorder of the Cp* group as well as disorder of the bridging SH group. Each was modeled in two positions with 50% occupancy. Soft restraints were applied to the thermal parameters and bond distances of the disordered atoms. All non-hydrogen atoms were refined anisotropically. The hydrosulfide hydrogen was located then fixed for refinement purposes. No decomposition was observed during data collection. Details of the data collection and refinement are given in Table 6. Further details are provided in the Supporting Information.
Acknowledgment. This work was supported by the U.S. Department of Energy’s (DOE) Office of Basic Energy Sciences, Chemical Sciences program. The Pacific Northwest National Laboratory is operated by Battelle for DOE. The Bruker (Siemens) SMART APEX diffraction facility was established at the University of Idaho with the assistance of the NSF-EPSCoR program and the M. J. Murdock Charitable Trust, Vancouver, WA. Supporting Information Available: Text giving additional details of the effects of hydrogen in the pKa measurements and CIF files giving X-ray data collection, refinement results, atomic coordinates, anisotropic displacement parameters, and complete bond distances and angles in CIF format for S4MeH and S4H2. This material is available free of charge via the Internet at http://pubs.acs.org. OM800875N
