Photochemistry of dithiocarbamate complexes of Ni(IV). I. Processes of photoreduction of complexes

A study of the soft X-ray photoreduction of [NiIV(S2CNEt2)3][BF4] and soft X-ray photoisomerisation of [PPh4][NiII(S2COEt)3] at the Ni L-edge †

“…After a detailed investigation of the changes occurring in the electronic absorption spectra, the photochemical changes were rationalised on the basis of the homolytic cleavage of two Ni-S bonds with the overall reduction of Ni IV to Ni II and oxidation of two dtc moieties to form a thiuramdisulfide (tds) ligand. 47 As the Ni L-edge spectra of [Ni IV (S 2 CNEt 2 ) 3 ][BF 4 ] have the same form after both visible and soft X-ray photochemistry, it is reasonable to assume that the same kind of process could be occurring in both cases, even though the visible photochemistry results from a ligand to metal charge transfer transition whereas the soft X-ray photochemistry arises from 2p → 3d transitions and photoionisation. Although the optical photochemical pathway involving homolytic cleavage of Ni-S bonds can be used to account for soft X-ray photoreduction in [Ni IV (S 2 -CNEt 2 ) 3 ][BF 4 ], unfortunately, consideration of the electron balance shows that this does not account for the change from octahedral Ni II to square-planar Ni II in [PPh 4 ][Ni II (S 2 COEt) 3 ].…”

“…This was interpreted to imply that the photochemical processes were occurring in an excited triplet state rather than in a singlet state. 47,48 To assess the effect of photon flux, spectra of [Ni IV (S 2 -CNEt 2 ) 3 ][BF 4 ] were recorded just before and directly after a fresh electron beam was introduced into the storage ring. This was the only practical means of reducing the total flux whilst maintaining the same spot size and resolution, as both of these would be affected by a change in the position of the exit slits.…”

“…The similarity of the Ni L-edge spectra obtained after both soft X-ray or visible irradiation of [Ni IV (S 2 CNEt 2 ) 3 ][BF 4 ] indicates that similar processes are probably at work in both cases, and implies that the same overall photochemistry may be occurring in the solid state with soft X-ray irradiation as with visible light. 47 A previously soft X-ray irradiated sample of [Ni IV (S 2 CNEt 2 ) 3 ][BF 4 ] showing a considerable extent of photoreduction was left in the dark (both visible and soft X-ray) for ca. 1 h. Subsequent scanning over the Ni L-edge at the same sample position showed no detectable reversion to the Ni IV state, implying the absence of the so-called 'dark reaction' previously observed when the dark colour of the Ni IV species returned to give a product spectroscopically identical to the starting material when the acetonitrile, ethanol and methanol solutions 47, 48 and solid PMMA matrices 48 were left in the dark.…”

“…However, it should be noted that no such 'dark reaction' was reported when frozen methanol matrices were used. 47…”

“…47,48 In solution,[Ni IV -(S 2 CNR 2 ) 3 ]Br (R = n Bu,42,47,48 CH 2 Ph 45 ) have been shown to be photoreduced to [Ni II (S 2 CNR 2 ) 2 ], NiBr 2 and the corresponding thiuramdisulfide (tds), with a dark reaction resulting in the re-formation of [Ni IV (S 2 CNR 2 ) 3…”