J. Am. Chem. SOC.1995,117,963-973
963
Study of Sulfinate and Sulfenate Complexes Derived from the Oxygenation of Thiolate Sulfur in [1 ,5-Bis(2-mercapto-2methylpropy1)- 1,5-diazacyclooctanato(2-)]nickel(II) Rizalia M. Buonomo, Ivan Font, Michael J. Maguire, Joseph H. Reibenspies, Thawatchai Tuntulani, and Marcetta Y. Darensbourg* Contribution of the Department of Chemistry, Texas A & M University, College Station, Texas 77843-3255 Received August 17, I994@
Abstract: A series of nickel(I1) complexes containing sulfinate and sulfenate S-donor sites has been prepared from systematic oxygenations of the dithiolate complex [1,5-bis(2-mercapto-2-methylpropyl)-1,5-diazacyclooctanato(2-)]nickel(II) ((bme*-daco)Ni, 1") and the whole series characterized by mass spectrometry and X-ray crystallography. The dithiolate complex reacts in organic solvents with molecular oxygen to produce sulfinate (NiS(0)2R) complexes, while reactions with hydrogen peroxide have, in addition, permitted the isolation of sulfenate (NiS(0)R) products. The X-ray crystal structures of the complexes [ l-(2-mercapto-2-methylpropyl)-5-(2-sulfino2-methylpropy1)- 1,5-diazacyclooctanato(2-)]nickel(II) (2*), [1,5-bis(2-sulfin0-2-methylpropyl)1,5-diazacyclooctanato(2-)]nickel(II) (3*), [ 1,5-bis(2-sulfen0-2-methylpropyl)-1,5-diazacyclooctanat0(2-)]Nckel(JZ) (S*), and [1-(Zsulfeno2-methylpropyl)-5-(2-sulfino-2-methylpropyl)-l,5-diazacyclooctanato(2-)]nickel(II) (a*) were determined and metric data compared with previously characterized'1 and 4*, [1-(2-mercapto-2-methylpropyl)-5-(2-sulfeno-2-methylpropyl)1,5-diazacyclooctanato(2-)]nickel(II). Analysis of the Ni-S(,, bond distances for homoleptic complexes reveals that the bond distances are in the order Ni-S(0)R (2.167(5)) > Ni-SR (2.159(3)) > Ni-S(O)ZR (2.127(3)). The average nickel sulfinate S-0 distance at 1.459 8, is ca. 0.1 8, shorter than the average nickel sulfenate S-0 distance at 1.545 8,. These bond dimensions correlated well with spectroscopic and reactivity data. Electrochemical studies find the Nim reversible couple to be stabilized by ca. 100 mV with conversion of thiolate-S to sulfenate-S donor, while conversion of a sulfenate to a sulfinate stabilized the Nil state by an additional 200 mV. Mass spectrometry delineated 0-atom loss pathways to dominate initial fragmentation pattems from the parent ions in the protic matrix thioglycerol; however, SO2 extrusion also occurs and is prominent in a nitrobenzyl alcohol ( M A ) matrix. The sulfenate complexes are unexpectedly stable and do not undergo either inter- or intramolecular disproportionation reactions to thiolate and sulfinate, nor do they transfer oxygen to triphenyl- or tributylphosphine. Sulfur dioxide removed oxygen from the sulfenates, yielding thiolates and SO3, whereas no reaction occurred with the sulfinate complexes. Complexes that represent partial oxidation of the sulfurs further reacted with hydrogen peroxide to produce higher oxygenates and also reacted with electrophiles such as alkyl halides to produce S-bound sulfoxide ligands. Introduction It has long been known that thiols (including cysteine) undergo aerial oxidation to form disulfides (cystine) and that this reaction is catalyzed by the presence of transition metals such as nickel.' Nickel-containing enzymes such as COdehydrogenase* and [NiFe] hydrogenase3 possess a sulfur-rich (cysteinate) environment around nickel and display interesting and complicated air sensitivity, which includes reversible inhibition of activity and irreversible deactivation of the enzyme by dioxygen. Electrochemical and spectroscopic evidence from enzyme studies supports the hypothesis that the deactivated species might involve oxidized Ni and also products of oxygen addition at sites other than the metal.3 The possible sulfur oxygenates are sulfinate ligands, or metallosulfones, M-SOzR, and sulfenates, or metallosulfoxides, M-S(=O)R. To date, Abstract published in Advance ACS Abstracts, December 1, 1994. (1) (a) The Chemistq and Biochemisby of the Sulfhydryl Group in Amino Acids, Peptides and Proteins; Friedman, M., Ed.; Pergamon Press: Oxford, U.K.,1973. (b) Takagi, T.; Isemura, T. J. Biochemistry 1964,54, 344. (c) Vinichenko, I. G.; Zegzhda, G. D.; Maiborada, 0. M. Rum. J. Inorg. Chem. 1985,997. (2)Shin, W.; Lindahl, P. A. Biochim. Biophys. Acta 1993, 1161, 317. (3) (a) van der Zwaan, J. W.; Coremans, J. M. C. C.; Bouwens, E. C.; Albracht, S. P. J. Biochim. Biophys. Acta 1990, 1041, 101. (b) Coremans, J. M. C. C.; Van der Zwann, J. W.; Albracht, S. P. J. Biochim. Biophys. Acta 1992, 1119, 157. @
there are two nickel thiolate complexes that have provided examples of the latter: whereas three potential thiolate model complexes have demonstrated production of sulfinates from reactions with H202 or gaseous 0 2 . Such NiS02R complexes are all S-bound and the oxygen atoms have not been removable by chemical agents. Hence an S-bound sulfinate could represent an irreversibly oxygen-damaged enzyme. No examples of 0-bound sulfinate ligands, of the type characterized by Zubieta et al. from the reaction of a ruthenium cis-dithiolate with dioxygen, have been found.5 Other research groups have reported results on the direct oxygenation of nickel thiolate complexes as models for oxygendegraded enzymes. Maroney6 et al. have shown that a nickel(II) complex with trans thiolates reacts with dioxygen to form a thiolatehlfinate complex, which is no further reactive with dioxygen. Schrauzer' has reacted dioxygen with an anionic (4) (a) Font, I.; Buonomo, R. M.; Reibenspies, J. H.; Darensbourg,M.
Y.Inorg. Chem. 1993, 32, 5897. (b) Amatore, C.; Darensbourg, M. Y.; Farmer, P. J.; Musie, G.; Verpeaux, J. N. J. Am. Chem. SOC. 1994, 116, 9355. ( 5 ) Nicholson, T.; Zubieta, J. Inorg. Chem. 1987,26, 2094. (6)Mizra, S. A.; Pressler, M. A.; Kumar, M.; Day, R. 0.;Maroney, M. J. Inorg. Chem. 1993, 32, 977. (7)Schrauzer, G. N.; Zhang, C.; Chadha, R. Inorg. Chem. 1990, 29, 4104.
0002-786319511517-0963$09.00/00 1995 American Chemical Society
964 J. Am. Chem. SOC.,Vol. 117, No. 3, 1995
Buonomo et al.
Scheme 1
-
2'
complexes were also is01ated.l~ Dioxygen was not used as an oxygen source in these reactions. Deutsch14 has characterized a cobalt sulfenate complex by crystallography and ascribes the scarcity of such complexes to the ability of the sulfenate ligand In addition to react as both an electrophile and a nu~leophi1e.l~ to the cobalt derivatives, sulfenate complexes were reported for p1atinum,l6 iridium," and possibly ruthenium,'* but not for nickel, outside of this research.
3
3*
nickel complex containing two sets of cis thiolate sites to produce a cis disulfinate complex. The remaining cis thiolate sites showed no further reaction with dioxygen nor with hydrogen peroxide. The nickel(II) dithiolate complexes based on the bis(mercaptoethy1)diazacyclowtanatoligand (1)* undergo aerial oxidation at sulfur, in organic solvents, to produce sulfinate complexes 2 and 3 (Scheme 1) rather than the conventional product of thiolate oxidation, the disulfide. A similar, but more sterically hindered, complex [ 1,5-bis(2-
mercapto-2-methylpropy1)-1,5-diazacyclooctanato(2-)]nickel(11) (l*,( b m e * - d a c ~ ) N i ) ~undergoes * ~ ~ ' ~ much slower reactions with molecular oxygen to produce mono- and disulfinate complexes 2* and 3*, respectively, and also will, as we report below, react with stoichiometric amounts of hydrogen peroxide to produce stable sulfenates 4*-6* shown in Scheme 1. One such sulfenate complex, 4*, was characterized structurally and reported in communication form.4 This series permits electrochemical studies to augment those of the (bme-daco)Ni complexes 1-3, which showed that the accessibility of the Nim couple depends on the substituents on the sulfur donors: SR2 > -S(0)2R > -SR . Results herein on complexes 1*-6*, will extend this knowledge to include sulfenates, -S(O)R, expected to be intermediate between thiolates and sulfinates in character. Earlier papers reported that the thiolate sulfurs of (bme-daco)Ni react with dioxygen in organic solvents via two separate paths to produce mono- or disulfinate complexes; in each case, dioxygen always adds as a molecule.'' Isotopic labeling studies12 indicated that a likely pathway to the disulfinate would involve the formation of a disulfenate (or metallosulfoxide) intermediate by dioxygen addition across cis sulfur sites; however, such a disulfenate Ni(S(=O)R)2 could not be isolated. The introduction of steric constraints a to the thiolate sulfurs in the (bme*-daco)Ni complexes has allowed the further stabilization and isolation of stable sulfenate complexes? Sulfenate complexes are uncommon and have been the target of rather limited research. Reports of stable cobalt(1II) complexes13 show that thiolate ligands may be converted to sulfmates by the action of hydrogen peroxide, and in some cases, sulfenate (8) Mills, D. K.; Reibenspies, J. H.; Darensbourg, M. Y . Inorg. Chem. 1990, 29, 4364. (9) Darensbourg, M. Y.; Font, I.; Pala, M.; Reibenspies, J. H. J. Coord. Chem. 1994, 32, 39. (10) Buonomo, R. M.; Darensbourg, M. Y.; Font, I. Maguire, M. J.; Reibenspies, J. H. J. Inorg. Biochem. 1993, 51, 343. (1 1) Farmer, P. J.; Solouki, T.; Soma, T.; Russell, D. H.; Darensbourg, M. Y. Inorg. Chem. 1993, 32, 4171. (12) Farmer, P. J.; Solouki, T.; Mills, D. K.; Soma, T.; Russell, D. H.; Reibenspies, J. H.; Darensbourg, M. Y. J. Am. Chem. SOC.1992,114,4601. (13) (a) Arnold, A. P.; Jackson, W. G. Inorg. Chem. 1990,29, 3618. (b) Dollimore, L. S.; Gillard, R. D. J. Chem. SOC., Dalton Trans. 1973, 933. (c) Jackson,W. G.; Sargeson, A. M.; Whimp, P. 0. J. Chem. SOC.,Chem. Commun. 1976, 934.
Experimental Section General Techniques. Complex 1* was prepared and further reacted with hydrogen peroxide, under anaerobic conditions, while subsequent handling of the sulfur oxygenates was performed in air. Isobutylene sulfideL9and 1,5-diazacyclooctaneZo(daco) were synthesized by published procedures. Hydrogen peroxide was used as a 30% w/v aqueous solution (Mallinkrodt). '*O-labeled dioxygen (99.8%) was purchased from Isotec. All other reagents were purchased from Aldrich Chemical Co. and used as obtained. Chromatography. All of the neutral nickel complexes were purified and separated from mixtures by chromatography through a 10 x 1 in. (or 0.5 in.) column of silica gel (60-200 mesh, Aldrich grade 922 or EM Science grade 22, pretreated with aqueous pH 10 buffer and washed with ethanol).zLMethanol (optimal), ethanol, and ethanovwater were used as eluants. Physical Measurements. NMR spectra were recorded using a Varian XL-200 FT NMR spectrometer. W-vis spectra were recorded by a Hewlett Packard HP8452A diode array spectrophotometer. Infrared spectra were recorded as KBr pellets using an IBM IW32 Fourier transform single beam spectrophotometer. Elemental analyses were performed by Galbraith Laboratories, Knoxville, TN. Mass spectra were determined at the Center for Chemical Characterization and Analysis, Texas A & M University. Positive ion fast atom mass spectra were recorded in thioglycerol and nitrobenzyl alcohol (NBA) matrices using a VG-70s spectrometer with a xenon source having particle energy of 10 keV. Data were collected by a VG11250J data system. Isotopomeric mass analyses were performed using a double laser light desorption (DLLD) technique in conjunction with either a linear time of flight (LTOF) or a Fourier transform ion cyclotron resonance (FTICR) method of ion detection. GC/MS of gas samples were recorded by a Hewlett Packard HP5995C with a quadrupole mass analyzer. Cyclic voltammographs were obtained using a Bio-Analytical Systems l00A electrochemicalanalyzer with a glassy carbon stationary electrode and a platinum wire auxiliary electrode. Samples were measured in acetonitrile solution (2.5 mM) with [n-Bu$J][PF6] (0.1 M) as the supporting electrolyte and a Ag/AgNOs reference electrode. Potentials were standardized with methyl viologen and referenced to NHE. All X-ray crystal structures were solved at the Crystal & Molecular Structure Laboratory Center for Chemical Characterization and Analysis at Texas A & M University. For complexes 2*-5*, X-ray crystallographic data were obtained on a Nicolet R3mN single crystal X-ray diffractometer operating at 55 kV and 30 mA, Mo K a ( I = 0.710 73 A) radiation, equipped with a Nicolet LT-2 cryostat. Diffractometer control software P3VAX 3.42 was supplied by Nicolet Analytical X-ray Instuments. Structures were solvedz2using SHELXT-PLUS. In all (14) (a) Adzamli, I. K.; Libson, K.; Lydon, J. D.; Elder, R. C.; Deutsch, E. Inorg. Chem. 1979,18,303. (b) Sloan, C. P.; Krueger, J. H. Inorg. Chem. 1975, 14, 1481. (15) Block, E.; O'Connor, J. J. Am. Chem. SOC. 1973, 95, 5048. (16) (a) Weigand, W.; Bosl, G. Z. Nafulforsch. 1992, 1165. (b) Weigand, W.; Bosl, G.; Robl, C.; Amrein, W. Chem. Ber. 1992, 125, 1047. (17) George, T. A,; Watkins, D. D. Inorg. Chem. 1973, 12, 398. (18) Johnson, M. D.; Nickerson, D. Inorg. Chem. 1992, 31, 3971. (19) Snyder, H. R.; Stewart, J. H.; Ziegler,J. B. J. Am. Chem. SOC.1947, 69, 2672. (20) Musker, W. K.; Hussain, M. S . Inorg. Chem. 1966, 5, 1416. (21) The current understanding of [(bme-dacoNi)zNiI2+formation is that it occurs primarily as a result of contact with acidic silica gel during chromatography. Acidification releases a NiZ+ ion which rapidly and irreversibly is complexed by two parent (bme-daco)Ni metallothiolate ligands. Base treatment of the silica gel prevents this side reaction.
Oxygenation of Thiolate Sulfir in Nickel Complexes cases, crystals were mounted on a glass fiber with epoxy cement at room temperature. In the structure determination of complexes 1* and 6*, a Rigaku AFC5 diffractometer was used, operating at 55 kV and 180 mA, Cu Ka (1 = 1.541 78 A) radiation. X-ray experimental conditions for all the complexes are shown in Table 3. Syntheses. The syntheses of complexes 1* and 4* have been described elsewhere?, 1J-Bis(2-mercapto-2-methylpropyl)-l,5-diazacyd~e. Isobutylene sulfide (3.00 g, 34 "01) was added in one portion to a solution of 1,5-diazacyclooctane (1.96 g, 17 mmol) in dry benzene (50 mL) stirred under nitrogen. After 48 h at room temperature, the solution was filtered through Celite to remove any precipitated disulfide and the solvent removed in vacuo, yielding a yellow oil (2.02 g, 40.8% yield). [1,5-Bis(2-mercapto-2-methylpropyl)-1,5-diazacyclooc~nato(2-)]nickel(II) (l*). The following is a revised version of the previously published method? 1,5-bis(2-mercapto-2-methylpropyl)1,5-diazacyclooctane (1.80 g, 6.2 mmol) was dissolved in dry toluene (100 mL). To this was cannulated slowly a green solution of Ni(acac)z (1.52 g, 5.9 mmol) in toluene (200 mL), producing a purple solution with a dark precipitate. The solution was filtered and the purple solid washed with toluene (2 x 100 mL), followed by diethyl ether (2 x 100 mL). The solid was dissolved in a minimum of methanol (5 mL) and purified by chromatography on a silica gel column (10 x 1 in.), eluting with ethanol (95%). A purple solid, 1* (0.20 g, 10% yield based on Ni(II)), resulted on evaporation of the solvent. Single crystals suitable for X-ray crystal structure analysis were obtained by ether diffusion of an acetonitrile solution. Mp: 298 "C dec. Anal. Calcd (found) for C1&&NiSz: C, 48.45 (47.72); H, 8.00 (7.86); N, 8.00 (7.83). [1-(2-Mercapto-2-methylpropyl)-5-(2-sulfipyl)l$-diazacyclooctanat(2-)]nickel(II) (2") and [1-(2-Sulfen0-2-meth-
ylpropyl)-5-(2-sulfino-2-methylpropyl)-1,5-diazacyclooctanato(2-)Jnickel(II) (6*). A purple solution of 1* (1.00 g, 2.88 mmol) in
J. Am. Chem. SOC., Vol. 117, No. 3, I995 965 (30% w/v aqueous solution) (16 yL,0.14 mmol) in methanol (20 mL) was added in 1 mL portions over a period of 2 h. The resulting brown solution was warmed to room temperature and purified by silica gel chromatography. The following products were separated, in order of elution, with ethanol: the starting dithiolate complex 1*, the monosulfenate complex 4* (50%) as an orange band, and the disulfenate complex 5* (18%) as a yellow-orange band. X-ray quality crystals of 4* and 5* were grown by ether diffusion into saturated dichloromethane solutions of 4* and 5*, respectively. For compound 4*, anal calcd (found) for C~&SNZS~ON~*HZO: C, 44.09(43.75); H, 7.87 (7.75); N, 7.34 (7.05). For compound 5*, three attempts to obtain elemental analyses consistent with the X-ray crystal structure determination were unsuccessful. This is ascribed to the hygroscopic nature of the oxygenates as well as the presence of Na+ from the base-treated separations column. The "best" analysis assumed anal calcd (found) for C14H28N2S~Ofii.2NaOH C, 36.62 (36.77); H, 6.59 (6.79); N, 6.10 (5.91). [l-(2-Sulfeno-2-methylpropyl)-5-(2-sulfi)-l~diiazacyclooctanato(2-)]nickel(II) (6*). The sulfinate/sulfenatecomplex 6* was obtained in 48% yield, as a partial product along with 2*, from the reaction of 1* (1.0 g, 2.9 m o l ) with H202 (0.64 mL, 5.6 m o l ) in MeOH (250 mL) at -78 "C. Compound 6* was recrystallized by ether diffusion into a concentrated dichloromethane solution. The molecular ion for this complex was observed in the FAB MS in an NBA matrix at mlz 396 (396 expected). Due to cocrystallization,elemental analyses were unacceptable. The crystal structure revealed that the crystal cell contained 71% of complex 6* and 29% of complex 3*. Reaction of 1* with 0 2 . Dioxygen was passed through a purple solution of 1* (0.1 g, 0.29 "01) dissolved in dry MeCN (20 mL) for a period of 24 h. The reaction mixture was then sealed and left under a positive 02 pressure for a further 72 h. The resulting brown solution was reduced in volume by evaporation of the solvent, and the products were separated by column chromatography on silica gel (IO x 1 in.). The products eluted (methanol) in the following order: the purple 1* (89 mg), the orange monosulfinate complex 2* (2 mg), and the yellow disulfinate complex 3* (9 mg). The disulfenate complex 5* also showed uptake of molecular oxygen, and this was followed by GCMS and 180-labeling studies. The complex 5* was shown to convert under bubbling oxygen in MeCN solution to the disulfinate complex 3*, the reaction having a half-life of 8 h. For the labeling study, 5* (2.4 mg, 6.4pmol) was dissolved in MeCN (1 mL) in a flask containing 45% dioxygen (6:l 1s02:1602, 0.5 mmol total). The product was isolated after 7 days and shown by FAT3 MS to be complex 3* that had incorporated one molecule of l S 0 2 per nickel.
methanol (250 mL) was cooled to -78 "C under nitrogen. A solution of hydrogen peroxide (30% wlv aqueous solution) (0.64 mL, 5.6 m o l ) in methanol (20 mL) was then added in 0.1 mL aliquots over a 2 h period. The resulting yellow solution was concentrated and purified by chromatography on a silica gel column with methanol elution. The monosulfmate complex 2* eluted fist as an orange band (0.56 g, 51%). The yield of 2* is higher when the reaction was performed in methanol at -78 "C than when the reaction was performed in water at room temperature. Single crystals of 2* suitable for X-ray analysis were grown by the slow evaporation of a concentrated MeCN solution. A summary of the crystallographic parameters for compound 2* is found in Table 3. Mp: 260 OC dec. Anal. Calcd (found) for C&~&S~OZNi: C, 44.36 (44.23); H, 7.39 (7.74); N, 7.39 (6.87). A second orange band was collected and identified as containing Results and Discussion the sulfiiatelsulfenatecomplex 6* (0.52 g, 48%). Details are presented below. Synthesis. The preparation of complex 1" and the nucleo[1,5-Bis(2-su~~o-2-methylpropyl)-1,5-diazacyclooc~nato(2-)]- philic activity of its sulfur sites have been reported previo~sly.~ nickel(I1) (3*). A solution of 1* (0.10 g, 0.14 m o l ) in water (20 In that report, the steric effect of the four methyls a to the mL) at 22 "C was reacted with excess hydrogen peroxide (30% w/v sulfurs was demonstrated by analysis of structural differences aqueous solution) (0.32 mL, 2.9 m o l ) added in one aliquot, resulting in macrocyclic complexes derived from 1* vs 1. in an immediate color change to bright yellow. The solution was stirred The sulfur oxygenates presented below were obtained by at room temperature for a further 2 h. Chromatography of the product reaction of the dithiolate complex 1* with hydrogen peroxide on a silica gel column (10 x 1 in.), eluting with 40% watedethanol, as described in the Experimental Section. The yields of and subsequent removal of the solvent by vacuum yielded the disulfiiate individual complexes were optimized by variation of reaction complex 3* as a bright yellow solid (0.072 g, 60%). Single crystals suitable for X-ray were grown from a HzO/acetone mixture. A stoichiometry,temperature, and dilution. Further studies of the summary of the crystallographic parameters for compound 3* is found reactivities of the sulfurs in the complexes with 0 2 and in Table 3. Mp: 250 "C. Anal. Calcd (found) for C~J-IZBNZS~O~N~: electrophiles are presented later. C,33.80 (32.53); H,5.63 (5.84); N, 7.88 (7.51). Infrared Studies. Table 1 lists the v ( S 0 ) absorptions for [1-(2-Mercapto-2-methyIpropyl)-5-(2-sulfeno-2-me~ylpropyl)the oxygenate complexes 2*-6*. The assignment of the bands 1,5-diazacyclooctanato(2-)]nickel(II) (4*) and [l,J-Bis(Z-sulfenoin the 900-1200 cm-' region for the title series to v(S0)was 2-methylpropyl)-1,5-diazacyclooctanato(2-)]~ckel(II) (5*). A soconfirmed by comparisons of the IR spectra of 180-labeled lution of l * (0.10 g, 0.29 m o l ) in methanol (250 mL) was cooled to representative complexes. For example, bands for complex 2" -78 "C under nitrogen, and a dilute solution of hydrogen peroxide (22) (a) Ibers, J. A., Hamilton, W. C., Eds.; Zntemarional Tables for X-ray Crysrallography; Kynoch: Birmingham,U.K., 1974; Vol. IV. (b) Sheldrick, G. M. SHELXT-PLUS, Institut fur Anorganische Chemie der Universitat, Gottingen, Germany.
typically seen at 1181 and 1054 cm-l were shifted to 1146 and 1019 cm-' in 180-labeled 2* when the complex was prepared by reaction with l8O2. The sulfinate complexes show a twoband set, v(SO),, and v(SO),, in the range 1190-1030 cm-',
966 J. Am. Chem. SOC.,Vol. 117, No. 3, 1995
Buonomo et al.
Table 1. Physical and Spectroscopic Properties of Compounds 1*-6*
1*
color IR" v ( S 0 ) (cm-') Ni-SO Ni-SOZ (asym) Ni-SOz (sym) UVb(nm (e))' LMCT d-d 'H NMRd,' (ppm)
FAB (mlz)[M
+ H]+f
Purple
352 (192) 486 (70) 1.42, s (12H)
347
KBr pellets. Measured in methanol solution.
6*
6*
orange
orange
yellow
909
92 1
918 1036 1182
360 (4697) 474 (534) 1.66, s (3H) 1.56, s (3H) 1.32, s (3H) 1.22, s (3H) 363
374 (5968) 472 (689) 1.51, s (6H) 1.47, s (6H)
368 (6717) 450 ( 1827) 1.60, s (3H) 1.37, s (3H) 1.17, s (3H) 1.16, s (3H) 395
3* orange
yellow
1044 1181
1032,1071 1180,1192
302 (6727) 422 (51) 1.37, s (6H) 1.25, s (6H)
320 (10 476) 380 (938) 1.27, s (12H)
41 1
379 E
in M-' cm-'.
whereas the sulfenates show v(S0) as single absorbances at lower wavenumber, ca. 900-920 cm-'. The lower frequency of the w(S0) stretches of the sulfenates as compared te that of their sulfinate rivals indicates a weaker S-0 bond in the sulfenate complexes5 and is consistent with X-ray crystal structure data, vide infra. A comparison of the v(S0) absorption frequencies for the sulfinate complexes 2* and 3" vs analogous 2 and 3 revealed only minor differences. lH NMR Studies. The previous series of oxygenates based on the bme-daco ligand displayed appreciable tetrahedral twistsz3 from the N2SzNi square plane, resulting in paramagnetism (for d8 nickel(I1)) which precluded NMR studies. The series of oxygenates based on the bme*-daco ligand are all diamagnetic, presumably due to a more rigorously square planar structure imposed by the substituent methyl groups on the carbon a to the sulfurs. Characterization of the complexes by NMR spectroscopy finds the methyl region of the 'H NMR spectra particularly diagnostic; chemical shift data for the methyl region of complexes 1*-6* are given in Table 1. The symmetry about the central nickel dictates the number of methyl resonances observed. At room temperature, all four methyl groups in the starting material, 1*, are equivalent and appear as a single resonance at 1.42 ppm (12H); whereas the disulfenate complex, 5*, displays two pairs of nonequivalent methyl groups and hence two singlets (6H) and the monosulfinate, 4*, has four nonequivalent singlet (3H) resonances, similarly to 6*. These simple symmetry guidelines can be used to identify the products. The low-temperature spectrum of 1* was expected to resolve the singlet (12H) into two singlets (6H), as the ligand's thermal motion slowed with respect to the NMR time scale and the axial and equatorial methyl groups became nonequivalent. However, the signal remained a singlet, with some broadening, at -78 "C in methanol solution. Mass Spectrometry. The complexes in the series of oxygenates 2*-6* were examined by FAB MS in both thioglycerol and nitrobenzyl alcohol matrices. All spectra show parent ions, and these are listed in Table 1. Additionally, all complexes show the presence of a stable fragment ion assignable to the dithiolate complex 1*, indicating that both sulfinate- and sulfenate-containing complexes undergo loss of oxygen. The path of oxygen loss from the ligands, however, is dependent (23) The tetrahedral twist angles are determined as the angle of the intersection of the normals of the S(l)-X-S(2) and N(l)-X-N(2) planes, where X is the centroid of the N& plane. As the nickel is not always equidistant from the nitrogens and the sulfurs, the centroid rather than the nickel is used to define the planes.
379
Methyl region. CD3OD, TMS reference. PThioglycerol matrix.
Scheme 2
1.
2,
363 (50) & 346(10)
379(100)
I
-=-%
- so7 - so2
314(80)
t
3*
411 (30)
4*
363 (70) & 346 (20)
5*
379 (75)
6*
395 (80)-0, 379 (36)-0, 362 (2) -0, 346 (2)---b 314 (30)
__*
346 (20)
314 (20)
4 363 (35) & 347 (1)
- so2
w+];
*For 1*, the molecular ion for oxygenates, [M matrix. Relative % in parentheses.
+ H+].
r
on both complex and matrix. For example, analysis of the sulfinate complex 3* in the thioglycerol matrix shows a stepwise loss of atomic oxygen from the molecular ion, yielding sulfenate intermediates, whereas in NBA loss of a fragment of mass 64 suggests SO2 is extruded (Scheme 2). Complex 2* readily loses SOz, showing a signal at m/z 314 whose great intensity suggests direct production from the parent ion rather than stepwise loss of two 0 and one S atom. These SO2 fragments were observed directly with use of the DLLD technique and were useful diagnostics in 1602/1802isotopic labeling studies.' The parent ions of the sulfenate complexes, 4*-6*, degrade initially by loss of atomic oxygen (Scheme 2). The magnitude of the signal at mlz 314 for complex 6" indicates SO2 loss from the remaining sulfinate moiety. The spectra of sulfenate complexes 4* and 5" do not show loss of SOz. Therefore the mechanism of oxygen loss from 5* is unlikely to involve disproportionation (or intramolecular 0-atom transfer) to the monosulfinate complex 2*.z4 Spectra of complexes 5* and 6*, (24) An alternate possibility for the production of ions of mlz 32 units lower could involve loss of dioxygen.
J. Am. Chem. SOC., Vol. 11 7, No. 3, I995 967
Oxygenation of Thiolate Sulfur in Nickel Complexes
I 179
5.
Figure 1. +FAB MS spectra of complexes 5* and 6*. Spectra recorded in NBA matrix.
representative of the fragmentation patterns for sulfenates and sulfinates, respectively, are shown in Figure 1. Under these conditions, there has been no indication of SO loss from any of the sulfenates. Furthermore, a DLLD MS study on the sulfinatelsulfenate complex 6* showed loss of SO;?at 64 Da but no signal at 48 Da that would relate to an SO fragment. Uptake of molecular oxygen ( 1 6 0 2 and l80;?) by the disulfenate complex 5*, to yield 3*, vide infra, was followed by FAB MS. The starting material 5" shows a molecular ion at mlz 379, and the disulfinate complex prepared from 5* and l60;? has a molecular ion at mlz 411. The product derived from addition of to 5* showed a molecular ion at mlz 415, indicating addition to yield doubly 180-labeled 3*. The presumption that l80;? added across the two sulfur sites of 5* was further supported by the fragmentation patterns produced by double laser light desorption mass spectrometry. Previous FTICR MS results of the isotopically labeled disulfinate complex of (bse-daco)Ni, 3, prepared from a mixture of l60;? and '*O;?,found S160180fragments, observed at 66 Da, indicating scission of the 0-0 bond across cis sulfur sites." In the latest study, complexes were examined using LTOF MS techniques after laser excitation. The disulfinate complex, 3*, formed by reaction of 1 8 0 2 (contaminated with l60;?) with the 160-disulfenatecomplex, 5*, showed S O 2 photofragment signals at mass 64 and 66 Da, corresponding to natural abundance SO;? and the expected mixed label S160180fragment, respectively. Cyclic Voltammetry. Cyclic voltammographs for complexes 1*, 3*, and 5" are shown in Figure 2, and a compilation of the
w
t1.00
-1.00
0.00
-3.00
-2.00
Potential W) Figure 2. Cyclic Voltammographs for complexes 1*, 3*, and 5*. Samples recorded as 2.5 mM solutions in 0.1 M TBAHFP/MeCN using a glassy carbon electrode at 200 mV s-l. Potentials referenced to NHE with MeV2+/MeV+.
Table 2. Potentials and Reversibility Data from Cyclic Voltammetry in CH3CNb for Compounds 1*-6* rev Niu/Nil compd
Eln (mV)
DE (mV)
1*
-2119 -1829 -1488 -2030 -1833 - 1720
74 76 68 71 69
2*
3* 4* 5* 6*
85
irrevox
iP&
Epa(mV)
0.95 0.88 0.89 0.75 0.71 0.80
+264 +574 $1290 $310.8 +845.9 +800.7
EPn(mV)
+318.5 -62.2 +294 +324
All potentials scaled to NHE referenced to a MeV+/MeV+ standard (ElnNHE= -440 mV). bO.l M TBAHFP electrolyte measured vs a Ag/AgNOs reference electrode.
electrochemical data for the series of complexes 1*-6* is displayed in Table 2. The electrochemical data for (bme*-daco)Ni (I*) and its derivatives are very similar to that for the analogous (bme-daco)Ni series.25 The cyclic voltammographs show reversible Nim reduction waves for all the complexes, and complexes that contain a nickel thiolate moiety show S-based irreversible
968 J. Am. Chem. Soc., Vol. 117, No. 3, 1995
Buonomo et al.
Table 3. CrvstallograDhic Data for ComDounds 1*-6* ~~~~~~~
complex chem. form. FW (amu)
~
2*
1*
:p;c (4 b
(4 v (A3)
c
CI~H~~NZO~SZ C14Hz9N201Ni N~
C~H33N204S2Ni
347.2 Pnma 11.307(2) 19.579(3) 7.4790( 10) 1655.7(4)
5Q1.3 P1 8.066(2) 10.159(3) 14.231(5) 11lO(6) 96.67(3) 105.70(3) 93.02(2) 2 296 5.4 (3.9)
424.3 P1 7.9210(2) 11.802(2) 12.0580(10) 1008.5(2) 105.760(10) 96.4OO( 10) 107.920(10) 2 296 5.0 (5.2)
379.3 Pbca (no. 61) 13.655(5) 13.004(4) 19.128(4) 3396(2)
a
Y (deg) 4 296 4.1 (4.1)
temp 6)
(a)
R (R,)"
4
"
"
8 296 6.8 (7.1)
375.7 P2( l)/c 10.643(4) 22.047(12) 7.912(3) 1749.2(13) 109.58(3) 4 193 6.14 (6.08)
Scheme 3
'
+--; 1: +t-~ 201 mV
1* 1'
5*
2*
6*
3*
I50
+050
-050
.
.
- I 50
-250
Potential (v) Figure 3. Square-wave voltammograph spectra for complexes 1*6*. Samples recorded as 2.5 mM solutions in 0.1 M TBAHFF'MeCN using a glassy carbon electrode at 200 mV s-l. Potentials referenced to NHE with MeV2+/h4eVf.
oxidations. On the basis of previous EPR identification of the Nil species in reduced 3,25the reversible reduction waves in all complexes are assigned to Ni". The Ni" couple for 1*, at an E112 of -2119 mV (vs NHE), is 175 mV more difficult to attain than that for (bme-daco)Ni (at - 1944 mV), accountable to the (25) Darensbourg, M. Y.; Farmer, P. J.; Lindahl, P. A,; Reibenspies, J. H.J. Am. Chem. SOC. 1993, 115, 4665.
C2/c (no. 15)
19.967(8) 7.970(2) 24.287(5) 107.44 8 296 6.0 (6.1)
109 mV
89,
197 mV
4*
6*
5*
CI~HZSN~SZN~ Cl&&O2S2Ni
B (de& Z
4*
3*
6. 232 "V,
3'
113 mV
fact that the methyl substituents of 1* increase the electron density on the sulfurs, which subsequently destabilizes the electron-rich Nil oxidation state. Since the bme*-daco free ligand has an irreversible oxidation at -+285 mV, the first of two irreversible oxidations (+264 and +lo74 mV) of complex 1* is assigned to ligand oxidation. This oxidation is 75 mV more accessible than for (bme-daco)Ni, again accountable to increased electron density at sulfur. The second oxidation wave of 1* is possibly metal-based, but attempts to isolate the NiIU state of 1* by chemical oxidation were unsuccessful. Complexes 2* and 3* have reversible Ni" couples at Ell2 of -1829 and -1488 mV (vs NHE), respectively. The Ni" couples for 2* and 3* are less accessible by 198 and 149 mV than their respective bme-daco complexes 2 and 3. The most notable difference between the (bme-daco)Ni and (bme*-daco)Ni systems is that complex 3* has an irreversible oxidation at +1290 mV rather than the reversible couple seen for 3 at +847 mV. Complex 2* has an irreversible oxidation wave at +574 mV. The effect of sulfenato ligands on nickel redox potentials was established by cyclic voltammetry of 4*-6". Table 2 summarizes the electrochemical data for the series, and the stacked square-wave plots for complexes 1*-6* in Figure 3 show the progression of accessibility of the Ni" couple with each 0 added to S , which is summarized in Scheme 3. The addition of one oxygen to complex 1" causes a stabilization of the Ni' oxidation state by 89 mV. Further addition of oxygen to complex 4* yields two distinct products, 2* and 5*. In both cases, the addition of a second oxygen either to the sulfenate (Ni-S(0)R) or to the second thiolate (Ni-SR) stabilizes the Ni' by approximately 200 mV. Addition of another oxygen atom to either 2* or 5" yields the same product, 6*. The NiIM stabilization obtained from either process amounts to -1 10 mV. The final oxygenation yields complex 3* with a stabilization of 232 mV for this step. In essence, even to odd changes in number of oxygens increase the accessibility of the Ni" couple by -100 mV, while odd to even changes are -200 mV, with an overall stabilization of Nil in going from 1* to 3" of ' 6 0 0 mV. These data validate the 300 mV (-6.9 kcal mol-') stabilization on the oxidation of a nickel-bound thiolate (Ni-SR) to a sulfinate (Ni-SO2R) first found for (bme-daco)Ni and its sulfinate derivative^.^^ These shifts in potential should be
Oxygenation of Thiolate Sulfur in Nickel Complexes
J. Am. Chem. SOC., Vol. 117, No. 3, 1995 969 Scheme 5
Scheme 4 1'
3'
5'
Scheme 6 a
b
C
accurate reflections of the difference in donor ability between the thiolate and the sulfinate ligands since the charge of the complex remains the same. Interestingly, the d-d transitions for complexes 1*-6* (Table 1) also agree that the SO2R ligands are stronger field ligands, with a similar progression of d-d splitting as seen in the stabilization of the electron-rich Nil. Although the nickel(I1) sulfenate complexes are thermally stable, repeated cyclic scans of complex 4* produced waves attributed to 1* and 2*. That is, the sulfenate complex is observed to disproportionate under electrochemical reducing conditions. This phenomenon has been studied in detail for complex 4 and is the subject of a separate Reactivity. Reactions with Molecular Oxygen. Thiolates: Complex 1*reacts slowly with dioxygen in acetonitrile solution under ambient conditions (Pyrex glassware, laboratory light, 22 "C) to produce the mono- and disulfinate complexes 2* and 3*. Over the course of 1 week, there is
