A diimine ligand, LL = 2-C5H4NCHN-4-C6H4NNPh, which carries a trans-azobenzene substituent, forms the dimethylplatinum(II) complex [PtMe2(LL)], which undergoes trans oxidative addition
with MeI, PhCH2Br, Br2, and I2 to
give the corresponding organoplatinum(IV) complexes [PtIMe3(LL)], [PtBrMe2(CH2Ph)(LL)], [PtBr2Me2(LL)], and [PtI2Me2(LL)], respectively.
The ligand and the platinum(II) and platinum(IV) complexes are shown
to undergo trans–cis isomerization of the
azobenzene substituent upon irradiation, and the cis isomers then underwent slow thermal isomerization back to the more
stable trans isomers.
The complexes [PtMe2(NN)] (NN = 2,2′-bipyridine
= bipy, 1a; NN = di-2-pyridylamine = dpa, 1b; NN = di-2-pyridyl ketone = dpk, 1c) react with dimethyldioxirane
in moist acetone to give the hydroxoplatinum(IV) complexes [Pt(OH)2Me2(NN)] (NN = bipy, 2a; NN = dpa, 2b, or [Pt(OH)Me2(dpkOH)], 3). Complex 2a crystallizes as the hydrate 2a·7H2O, which has a complex supramolecular network structure formed
through hydrogen bonding between PtOH groups and water molecules.
Attempts to trap a potential oxoplatinum(IV) intermediate in these
reactions were unsuccessful, and computational studies suggest that
oxoplatinum(IV) intermediates are improbable. It is suggested that
oxygen atom transfer from the dioxirane to platinum is coupled to
proton addition to give the hydroxoplatinum group directly.
Several platinum(ii) complexes with ligands containing azo groups have been prepared and structurally characterised, and their photoswitching between trans and cis azo group isomers has been studied. The azo groups in the cationic complexes [PtMe(bipy)(4-NCH-N[double bond, length as m-dash]N-4-CHX)][PF], X = H, OH or NMe, and in the dicationic complex [Pt(bipy)(4-HNCH-N[double bond, length as m-dash]N-CH)][OTf] undergo trans to cis photoswitching on irradiation at 365 nm. The complex [PtMe(bipy)(4-NCH-N[double bond, length as m-dash]N-4-CHNMe)][PF] also exhibits a reversible halochromic effect on protonation to give the dicationic complex [PtMe(bipy)(4-NCH-NH[double bond, length as m-dash]N-4-CHNMe]. The nature of the frontier orbitals in the platinum(ii) complexes depends on the charge on the complex and on the degree of metal-ligand π-bonding.
The
ligand 1,4-di-2-pyridyl-5,6,7,8,9,10-hexahydrocycloocta[d]pyridazine (6-dppd) contains two potential chelate groups,
but it coordinates to only one dimethylplatinum group in forming the
complex [PtMe2(6-dppd)], 1. Complex 1 contains a free pyridyl and a free pyridazine nitrogen donor,
but it fails to coordinate to ZnCl2 or to CuCl to give
a bimetallic complex. Complex 1 reacted with mercury(II)
salts HgX2 (X = Cl, Br, OAc), not by coordination but by
oxidative addition to the dimethylplatinum(II) center to give the
platinum(IV) complexes [PtX(HgX)Me2(6-dppd)]. Complex 1 reacted with bromine or iodine by trans oxidative addition to give [PtX2Me2(6-dppd)],
X = Br or I, but, when X = I, a more complex sequence of reactions
also gave rise to products of cis oxidative addition
and to the products [PtIMe3(6-dppd)] and [PtI3Me(6-dppd)], which arise through a methyl group transfer reaction.
Complex 1 reacted with alkyl halides RX by trans oxidative addition to give [PtXRMe2(6-dppd)], R = Me,
X = I; R = CH2Ph, X = Br; R = CH2-4-C6H4-CH2Br, X = Br. The cleavage of methyl groups
from complex 1 by DCl gave a mixture of all isotopomers
of methane, CH4–n
D
n
, indicating ready equilibration between hydrido(methyl)platinum(IV)
and methaneplatinum(II) complex intermediates.
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