Electronic absorption, magnetic circular dichroism, and resonance Raman spectroscopies have been used to determine the nature of oxomolybdenum-thiolate bonding in (PPh4)[MoO(SPh)4] (SPh = phenylthiolate) and (HNEt3)[MoO(SPh-PhS)2] (SPh-PhS = biphenyl-2,2'-dithiolate). These compounds, like all oxomolybdenum tetraarylthiolate complexes previously reported, display an intense low-energy charge-transfer feature that we have now shown to be comprised of multiple S-->Mo dxy transitions. The integrated intensity of this low-energy band in [MoO(SPh)4]- is approximately twice that of [MoO(SPh-PhS)2]-, implying a greater covalent reduction of the effective nuclear charge localized on the molybdenum ion of the former and a concomitant negative shift in the Mo(V)/Mo(IV) reduction potential brought about by the differential S-->Mo dxy charge donation. However, this is not observed experimentally; the Mo(V)/Mo(IV) reduction potential of [MoO(SPh)4]- is approximately 120 mV more positive than that of [MoO(SPh-PhS)2]- (-783 vs -900 mV). Additional electronic factors as well as structural reorganizational factors appear to play a role in these reduction potential differences. Density functional theory calculations indicate that the electronic contribution results from a greater sigma-mediated charge donation to unfilled higher energy molybdenum acceptor orbitals, and this is reflected in the increased energies of the [MoO(SPh-PhS)2]- ligand-to-metal charge-transfer transitions relative to those of [MoO(SPh)4]-. The degree of S-Mo dxy covalency is a function of the O identical to Mo-S-C dihedral angle, with increasing charge donation to Mo dxy and increasing charge-transfer intensity occurring as the dihedral angle decreases from 90 to 0 degree. These results have implications regarding the role of the coordinated cysteine residue in sulfite oxidase. Although the O identical to Mo-S-C dihedral angles are either approximately 59 or approximately 121 degrees in these oxomolybdenum tetraarylthiolate complexes, the crystal structure of the enzyme reveals an O identical to Mo-SCys-C angle of approximately 90 degrees. Thus, a significant reduction in SCys-Mo dxy covalency is anticipated in sulfite oxidase. This is postulated to preclude the direct involvement of coordinated cysteine in coupling the active site into efficient superexchange pathways for electron transfer, provided the O identical to Mo-SCys-C angle is not dynamic during the course of catalysis. Therefore, we propose that a primary role for coordinated cysteine in sulfite oxidase is to statically poise the reduced molybdenum center at more negative reduction potentials in order to thermodynamically facilitate electron transfer from Mo(IV) to the endogenous b-type heme.

An attractive model: The iron complex shown on the left models the 2‐His‐1‐carboxylate active sites of Rieske dioxygenases both in terms of structure and function. 18O‐labeling studies of olefin dihydroxylation support the involvement of a high‐valent iron‐oxo species.

Four copper(I) tris(pyrazolyl)hydroborate complexes are reported with the fairly bulky tris[3-(p-tert-butylphenyl)-5-methylpyrazol-1-yl]hydroborate ligand (Tp()t(Bu-)(Ph,Me)). Tp()t(Bu-)(Ph,Me)Cu(CH(3)CN) (1) was synthesized from CuCl and Tp()t(Bu-)(Ph,Me)Li(CH(3)CN). The acetonitrile ligand in 1 was easily replaced by CO, PPh(3), and P(t)()Bu(3), forming Tp()t(Bu-)(Ph,Me)Cu(CO) (2), Tp()t(Bu-)(Ph,Me)Cu(PPh(3)) (3), and Tp()t(Bu-)(Ph,Me)Cu(P(t)()Bu(3)) (4), respectively. Complexes 1-4 have been crystallographically characterized. 1.4CH(3)CN, 173 K: C(52)H(67)BCuN(11), triclinic, P&onemacr;, a = 13.4201(10) Å, b= 15.132(2) Å, c = 15.2125(13) Å, alpha = 60.743(6) degrees, beta = 73.211(4) degrees, gamma = 74.839(5) degrees, Z = 2, R1 = 6.81% (wR2 = 18.91%). 2, 296 K: C(43)H(52)BCuN(6)O, monoclinic, C2/c, a = 25.592(4) Å, b = 12.434(2) Å, c = 28.044(3) Å, beta = 104.073(9) degrees, Z = 8, R1 = 7.47% (wR2 = 22.08%). 3.CH(2)Cl(2), 173 K: C(61)H(69)BCl(2)CuN(6)P, triclinic, P&onemacr;, a= 12.5080(13) Å, b = 15.159(3) Å, c = 17.151(2) Å, alpha = 64.271(10) degrees, beta = 79.073(7) degrees, gamma = 86.572(8) degrees, Z = 2, R1 = 5.13% (wR2 = 13.28%). 4.0.5 hexane, 298 K: C(57)H(86)BCuN(6)P, triclinic, P&onemacr;, a = 13.337(2) Å, b = 13.435(2) Å, c = 17.386(2) Å, alpha = 88.371(7) degrees, beta = 71.863(8) degrees, gamma = 80.223(9) degrees, Z = 2, R1 = 6.96% (wR2 = 18.62%). The Tp()t(Bu-)(Ph,Me) ligands in 1, 2, and 3 bind in a tridentate fashion; the CH(3)CN and CO ligands fit comfortably within the pocket formed by the tert-butylphenyl substituents and the PPh(3) ligand interleaves between the pyrazole arms. The flexibility of the pocket was probed by calculating the area of the triangle created by connecting the midpoints of the 3-phenyl groups; this parameter increases by 15% for 3 (the largest) over 1 (the smallest). Thus, the pocket exhibits some flexibility, found to be due to both steric and electronic factors. Complex 4 features a bidentate Tp()t(Bu-)(Ph,Me) ligand as the P(t)()Bu(3) apparently exceeds the pocket's flexibility.

A new macrocyclic ligand with a pendant naphthalene group, N-[2-(1-naphthyl)ethyl]-1-aza-4,8-dithiacyclodecane (L), has been synthesized and characterized. The copper(I)-acetonitrile complex [LCu(CH(3)CN)](PF(6)) (1) was synthesized from L and [Cu(CH(3)CN)(4)](PF(6)). The acetonitrile ligand from 1 was easily removed to give [LCu](PF(6)) (2). Complexes 1 and 2 have been crystallographically characterized. 1: C(21)H(28)N(2)CuF(6)PS(2), triclinic, P&onemacr;, a = 11.1901(10) Å, b = 11.2735(12) Å, c = 12.1350(10) Å, alpha = 98.996(8) degrees, beta = 117.188(6) degrees, gamma = 105.354(7) degrees, Z = 2, R1 = 0.0505 (wR2 = 0.1418). 2.0.5hexane: C(22)H(31)NCuF(6)PS(2), monoclinic, P2(1)/c, a = 15.7318(15) Å, b = 8.9164(10) Å, c = 17.205(5) Å, beta = 102.431(6) degrees, Z = 4, R1 = 0.0587 (wR2 = 0.1545). In addition, a cocrystallized mixture of both complexes was crystallographically characterized. 1&2.hexane: C(46)H(61)N(3)Cu(2)F(12)P(2)S(4), triclinic, P&onemacr;, a = 10.8308(9) Å, b = 12.6320(8) Å, c = 19.9412(13) Å, alpha = 80.445(5) degrees, beta = 76.405(6) degrees, gamma = 78.825(5) degrees, Z = 2, R1 = 0.0661 (wR2 = 0.1871). The solid-state structure of 2 features the pendant naphthalene group bound in an eta(2)-fashion, which is highly unusual for copper complexes. In CDCl(3), 2 exhibits fluxional behavior with the barrier to the process estimated, DeltaG() = 12-13 kcal. Variable temperature NMR spectroscopy gave compelling evidence for solution binding of the naphthalene group in 2, apparently the first example for copper(I). The fluxional process seen for 1 is best described as interconversion of the two enantiomers via a species with an unbound naphthalene group. Consistent with the weak binding of the naphthalene group, it is readily replaced with other ligands, such as triphenylphosphine to form [LCu(PPh(3))](PF(6)) (3). Complex 3 has also been structurally characterized: C(37)H(40)NCuF(6)P(2)S(2), monoclinic, P2(1)/c, a = 11.462(2) Å, b = 15.972(2) Å, c = 19.835(9) Å, beta = 94.50(3) degrees, Z = 4, R1 = 0.0906 (wR2 = 0.1889).

Mn(II)-dependent 3,4-dihydroxyphenylacetate 2,3-dioxygenase (MndD) is an extradiol-cleaving catechol dioxygenase from Arthrobacter globiformis that has 82% sequence identity to and cleaves the same substrate (3,4-dihydroxyphenylacetic acid) as Fe(II)-dependent 3,4-dihydroxyphenylacetate 2,3-dioxygenase (HPCD) from Brevibacterium fuscum. We have observed that MndD binds the chromophoric 4-nitrocatechol (4-NCH(2)) substrate as a dianion and cleaves it extremely slowly, in contrast to the Fe(II)-dependent enzymes which bind 4-NCH(2) mostly as a monoanion and cleave 4-NCH(2) 4-5 orders of magnitude faster. These results suggest that the monoanionic binding state of 4-NC is essential for extradiol cleavage. In order to address the differences in 4-NCH(2) binding to these enzymes, we synthesized and characterized the first mononuclear monoanionic and dianionic Mn(II)-(4-NC) model complexes as well as their Fe(II)-(4-NC) analogs. The structures of [(6-Me(2)-bpmcn)Fe(II)(4-NCH)](+), [(6-Me(3)-TPA)Mn(II)(DBCH)](+), and [(6-Me(2)-bpmcn)Mn(II)(4-NCH)](+) reveal that the monoanionic catecholate is bound in an asymmetric fashion (Delta r(metal-O(catecholate))=0.25-0.35 A), as found in the crystal structures of the E(.)S complexes of extradiol-cleaving catechol dioxygenases. Acid-base titrations of [(L)M(II)(4-NCH)](+) complexes in aprotic solvents show that the p K(a) of the second catecholate proton of 4-NCH bound to the metal center is half a p K(a) unit higher for the Mn(II) complexes than for the Fe(II) complexes. These results are in line with the Lewis acidities of the two divalent metal ions but are the opposite of the trend observed for 4-NCH(2) binding to the Mn(II)- and Fe(II)-catechol dioxygenases. These results suggest that the MndD active site decreases the second p K(a) of the bound 4-NCH(2) relative to the HPCD active site.

The reaction of Mo(O)2(acac)2, H2L (2,2'-dimercaptobiphenyl), and NEt3 produced the mononuclear Mo(V) complex Et3NH[Mo(O)(L)2] (1). Molybdenum mono-oxo tetrathiolate complexes such as 1 are studied as potential structural or functional models for pyranopterin-containing molybdoenzymes. Complex 1 has been crystallographically characterized. The solid-state structure reveals that the molybdenum ion sits within a cleft formed by the biphenyl backbone of the ligands, providing some steric protection. In addition, there is a hydrogen bond between the amine hydrogen of [Et3NH]+ and one of the thiolate sulfur atoms. A difference in solution reactivity between 1 and a derivative without a hydrogen-bonding counterion suggests that hydrogen bonding occurs in solution also. There are two short S-S contacts and small S-Mo-S angles in the structure of 1 that may reflect a slight bonding interaction. Such short S-S distances and small angles have been found in a couple of other Mo-thiolate complexes and in many of the molybdoenzyme crystal structures. Further characterization of 1 by EPR, IR, and UV-vis spectroscopies, as well as by cyclic voltammetry, is discussed and compared to known Mo(V)-oxo-tetrathiolate complexes as well as to relevant molybdoenzyme data. Reactions to generate Mo(VI) complexes from 1 resulted in net oxidation at the ligand to form its disulfide derivative, which dissociated from the metal center. This result suggests that modifications to the ligand to prevent this process are needed.

Ein attraktives Modell: Der links gezeigte Eisenkomplex modelliert die 2‐His‐1‐carboxylat‐Umgebung des aktiven Zentrums von Rieske‐Dioxygenasen hinsichtlich der Struktur und der Funktion. 18O‐Markierungsstudien der Olefin‐Dihydroxylierung sprechen für die Beteiligung einer hochvalenten Eisen‐Oxo‐Spezies.

The syntheses of two new ligands containing macrocyclic [10]-aneNS 2 (1-aza-4,8-dithiacyclodecane) units are reported, including the N-methylated analogue (L Me ) and a dinucleating version that separates two [10]-aneNS 2 groups with a m-xylyl spacer (L 2 ). Copper complexes with these new ligands as well as previously reported related complexes have been found to mediate the aziridination of olefins. Thus, isolated copper complexes containing the ethylnaphthyl-appended [10]-aneNS 2 macrocyclic ligand (L nap ), including [L nap Cu]PF 6 (1) and [L nap Cu(CH 3 CN)]PF 6 (2), the Cu(I) complex [L Me Cu(CH 3 CN)]PF 6 (3), and the Cu(II) complex L Me CuBr 2 (4), were compared in their ability to function as aziridination precatalysts. In addition, the aziridination capabilities were probed for complexes generated in situ from copper(I) ion sources and L 2 , 1,4,7-triazacyclononane, 1,4,7-trithiacyclononane, or 1,4,7trimethyl-1,4,7-triazacyclononane. The synthesis and characterization of the new complexes 3 and 4 are reported, including X-ray crystal structures. The aziridination reaction using precatalyst 3 was examined for its tolerance to different functional groups near the olefin as well as to the use of other nitrogen group sources and reaction conditions.