Synthesis and spectroscopic identification of a mu-1,2-disulfidodinickel complex

Cooperative Sulfur Transformations at a Dinickel Site: A Metal Bridging Sulfur Radical and Its H-Atom Abstraction Thermochemistry

Starting from the dinickel(II) dihydride complex [ML(Ni-H)2] (1M), where L3- is a bis(tridentate) pyrazolate-bridged bis(β-diketiminato) ligand and M+ is Na+ or K+, a series of complexes [KLNi2(S2)] (2K), [MLNi2S] (3M), [LNi2(SMe)] (4), and [LNi2(SH)] (5) has been prepared. The μ-sulfido complexes 3M can be reversibly oxidized at E1/2 = -1.17 V (in THF; vs Fc+/Fc) to give [LNi2(S•)] (6) featuring a bridging S-radical. 6 has been comprehensively characterized, including by X-ray diffraction, SQUID magnetometry, EPR and XAS/XES spectroscopies, and DFT calculations. The pKa of the μ-hydrosulfido complex 5 in THF is 30.8 ± 0.4, which defines a S-H bond dissociation free energy (BDFE) of 75.1 ± 1.0 kcal mol-1. 6 reacts with H atom donors such as TEMPO-H and xanthene to give 5, while 5 reacts with 2,4,6-tri(tert-butyl)phenoxy radical in a reverse H atom transfer to generate 6. These findings provide the first full characterization of a genuine M-(μ-S•-)-M complex and provide insights into its proton-coupled electron transfer (PCET) reactivity, which is of interest in view of the prominence of M-(μ-SH/μ-S)-M units in biological systems and heterogeneous catalysis.

Nonheme binuclear transition metal complexes with hydrosulfide and polychalcogenides

The intriguing chemistry of chalcogen (S, Se)-containing ligands and their capability to bridge multiple metal centres have resulted in a plethora of reports on transition metal complexes featuring hydrosulfide (HS-) and polychalcogenides (En2-, E = S, Se). While a large number of such molecules are strictly organometallic complexes, examples of non-organometallic complexes featuring HS- and En2- with N-/O-donor ligands are relatively rare. The general synthetic procedure for the transition metal-hydrosulfido complexes involves the reaction of the corresponding metal salts with HS-/H2S and this is prone to generate sulfido bridged oligomers in the absence of sterically demanding ligands. On the other hand, the synthetic methods for the preparation of transition metal-polychalcogenido complexes include the reaction of the corresponding metal salts with En2- or the two electron oxidation of low-valent metals with elemental chalcogen, often at an elevated temperature and/or for a long time. Recently, we have developed new synthetic methods for the preparation of two new classes of binuclear transition metal complexes featuring either HS-, or Sn2- and Sen2- ligands. The new method for the synthesis of transition metal-hydrosulfido complexes involved transition metal-mediated hydrolysis of thiolates at room temperature (RT), while the method for the synthesis of transition metal-polychalcogenido complexes involved redox reaction of coordinated thiolates and exogenous elemental chalcogens at RT. An overview of the synthetic aspects, structural properties and intriguing reactivity of these two new classes of transition metal complexes is presented.

Generation and Reactivity of Polychalcogenide Chains in Binuclear Cobalt(II) Complexes

A series of six binuclear Co(II)-thiolate complexes, [Co2(BPMP)(S-C6H4-o-X)2]1+ (X = OMe, 2; NH2, 3), [Co2(BPMP)(μ-S-C6H4-o-O)]1+ (4), and [Co2(BPMP)(μ-Y)]1+ (Y = bdt, 5; tdt, 6; mnt, 7), has been synthesized from [Co2(BPMP)(MeOH)2(Cl)2]1+ (1a) and [Co2(BPMP)(Cl)2]1+ (1b), where BPMP1- is the anion of 2,6-bis[[bis(2-pyridylmethyl)amino]methyl]-4-methylphenol. While 2 and 3 could allow the two-electron redox reaction of the two coordinated thiolates with elemental sulfur (S8) to generate [Co2(BPMP)(μ-S5)]1+ (8), the complexes, 4-7, could not undergo a similar reaction. An analogous redox reaction of 2 with elemental selenium ([Se]) produced [{Co2(BPMP)(μ-Se4)}{Co2(BPMP)(μ-Se3)}]2+ (9a) and [Co2(BPMP)(μ-Se4)]1+ (9b). Further reaction of these polychalcogenido complexes, 8 and 9a/9b, with PPh3 allowed the isolation of [Co2(BPMP)(μ-S)]1+ (10) and [Co2(BPMP)(μ-Se2)]1+ (11), which, in turn, could be converted back to 8 and 9a upon treatment with S8 and [Se], respectively. Interestingly, while the redox reaction of the polyselenide chains in 9a and 11 with S8 produced 8 and [Se], the treatment of 8 with [Se] gave back only the starting material (8), thus demonstrating the different redox behavior of sulfur and selenium. Furthermore, the reaction of 8 and 9a/9b with activated alkynes and cyanide (CN-) allowed the isolation of the complexes, [Co2(BPMP)(μ-E2C2(CO2R)2)]1+ (E = S: 12a, R = Me; 12b, R = Et; E = Se: 13a, R = Me; 13b, R = Et) and [Co2(BPMP)(μ-SH)(NCS)2] (14), respectively. The present work, thus, provides an interesting synthetic strategy, interconversions, and detailed comparative reactivity of binuclear Co(II)-polychalcogenido complexes.

Synthesis, structural characterization and conversion of dinuclear iron-sulfur clusters containing the disulfide ligand: [Cp*Fe(μ-η2:η2-bdt)(cis-μ-η1:η1-S2)FeCp*], [Cp*Fe(μ-S(C6H4S2))(cis-μ-η1:η1-S2)FeCp*], and [{Cp*Fe(bdt)}2(trans-μ-η1:η1-S2)]

The treatment of [Cp*Fe(μ-η²:η⁴-bdt)FeCp*] (1, Cp* = η⁵-C₅Me₅, bdt = benzene-1,2-dithiolate) with 1/4 equiv. of elemental sulfur (S₈) gave a dinuclear iron-sulfur cluster [Cp*Fe(μ-η²:η²-bdt)(cis-μ-η¹:η¹-S₂)FeCp*] (2), which contains a cis-1,2-disulfide ligand. When complex 2 further interacted with 1/8 equiv. of S₈, another sulfur atom inserted into an Fe-S bond to give a rare product [Cp*Fe(μ-S(C₆H₄S₂))(cis-μ-η¹:η¹-S₂)FeCp*] (3). Unexpectedly, a trans-1,2 disulfide-bridged diiron complex [{Cp*Fe(bdt)}₂(trans-μ-η¹:η¹-S₂)] (4) was isolated from the reaction of complex 1 with 1/2 equiv. of S₈, which represents a structural isomer of [2Fe-2S] ferredoxin-type clusters. In addition, cis-1,2-disulfide-bridged complex 3 can slowly convert into trans-1,2-disulfide-bridged complex 4 and the complex [Cp*Fe(μ-η²:η²-S₂)(cis-μ-η¹:η¹-S₂)FeCp*] (5) by self-assembly reaction at ambient temperature, which is evidenced by time-dependent ¹H NMR spectroscopy.

C-H activation by a diselenido dinickel(II) complex

Addition of selenium to the nickel(I) complex, [Ni(Me4[12]aneN4)(CO)]PF6, effects a redox reaction leading to the diselenido dinickel(II) complex, {[(Ni(Me4[12]aneN4)]2(Se2)}(PF6)2, in 70% crystalline yield. The product's structure features a μ-η(2):η(2)-Se2 ligand with Se-Se bond length of 2.379(13) Å. Upon mild heating, {[(Ni(Me4[12]aneN4)]2(μ-η(2):η(2)-Se2)}(PF6)2 oxidizes 9,10-dihydroanthracene or 1,4-cyclohexadiene forming the terminal hydroselenide, [Ni(Me4[12]aneN4)(SeH)]PF6, and anthracene or benzene, respectively. [Ni(Me4[12]aneN4)(SeH)]PF6 cleanly converts back to the diselenido dinickel(II) adduct upon addition of a phenoxy radical.

(Mu-eta2:eta2-disulfido)dinickel(II) complexes supported by 6-methyl-TPA ligands

A series of (mu-eta(2):eta(2)-disulfido)dinickel(II) complexes 2(n) have been synthesized by the reaction of Na(2)S(2) and nickel(II) complexes 1(n) supported by tris[(pyridin-2-yl)methyl]amine (TPA; 1(0)) [(6-methylpyridin-2-yl)methyl]bis[(pyridin-2-yl)methyl]amine (Me(1)TPA; 1(1)), bis[(6-methylpyridin-2-yl)methyl][(pyridin-2-yl)methyl]amine (Me(2)TPA; 1(2)) and tris[(6-methylpyridin-2-yl)methyl]amine (Me(3)TPA; 1(3)), respectively, and characterised by UV-vis and resonance Raman spectroscopy. X-ray crystallographic analyses on 2(2) and 2(3) supported by Me(2)TPA and Me(3)TPA, respectively, have revealed that the Ni(2)S(2) core is largely bent (approximately 30 degrees) along the S-S axis, being in sharp contrast to the planar Ni(2)S(2) core structure of the (mu-eta(2):eta(2)-disulfido)dinickel(II) complexes reported so far. The UV-vis spectra of 2(0) and 2(1) supported by TPA and Me(1)TPA, respectively, exhibiting an intense absorption band at approximately 360 nm together with a shoulder around 400 nm and a weak and broad absorption band around 450-600 nm, are very close to those of 2(2) and 2(3), suggesting that 2(0) and 2(1) also exhibit a similar distorted Ni(2)S(2) core structure. Resonance Raman spectra of 2(n) showed a characteristic S-S stretching vibration mode at approximately 450 cm(-1) with an isotope shift Delta nu((32)S-(34)S) = 10-15 cm(-1). The reaction of 2(n) with (p-Me-C(6)H(4))(3)P gave (p-Me-C(6)H(4))(3)P=S quantitatively based on 2(n). Hammett analysis on the sulfur atom transfer process from 2(n) to the phosphine derivatives [(p-X-C(6)H(4))P; X = OMe, Me, H and Cl] has indicated that the reaction involves an electrophilic ionic mechanism. Moreover, the order of reactivity of 2(n) toward PPh(3) has been found as 2(0) > 2(3) > 2(1) > 2(2). On the basis of these results, the ligand effects of Me(n)TPA on the structure and reactivity of the nickel(II) complexes have been discussed.