Cyclometallation of polydentate ligands containing pyrazole groups, including the synthesis of platinum(IV) complexes with tripodal [N-C-N]- ligand systems

Abstract

Dimethylplatinum(I1) complexes, PtR2(L), have been made by reaction of [PtR2(mu-SEt2)]2 with a range of polydentate nitrogen donor ligands containing one or more pyrazol-l-y1 (pz) donor groups, including the new ligand bis(pyrazol-1-yl)(thien-2-yl)methane. The complexes give cis-PtMe2(py)2 when dissolved in pyridine at ambient temperature, except for PtMe2(L) (L = (pz)2CH2, (pz)2C(H)Ph, (pz)3CH, or (pz)2(mim)CH (mim = N-methylimidazol-2-yl)), which undergo cyclometallation at a C(5) position of one pyrazol-l-y1 ring. The cyclometallated ligands have been examined as ‘isoelectronic’ analogues of nitrogen donor poly(pyrazol-1-yl)alkane and poly(pyrazol-l-yl)borate ligands. A carbon monoxide derivative, PtMe{(pz)2(C3H2N2)CH-N,C}(CO) (Id) and a series of phosphine complexes have been prepared. The complex PtMe{(pz)2(C3H2N2)CH-N,C}(py) (la) and polymeric [PtMe{(pz)2(C3H2N2)CH}]n (2) undergo oxidative addition reactions with organohalides to give the platinum(IV) complexes [fac-PtMe(R){(pz)2(C3H2N2)CH-N,N’,C}(py)]X (7a-d) and fac-PtXMe(R){(pz)2(C3H2N2)CH-N,N’,C} (8a,b), respectively. The new reagent 1-bromo-2- (pyrazol-1-yl)ethane forms [fac-PtMe{(pz)2(C3H2N2)CH-N,N’,C}(pzCH2CH2-N,C)]Br (lo), containing both [N-C-N]- and [N-C]- ligands. Reaction of Me1 or PhCH2Br with PtMe2{(pz)3CH} gives fac-PtXMe(R){(pz)2(C3H2N2)CH-N,N’,C} (8a,b) directly, and PtMe2{(pz)4C} undergoes a similar cyclometallation/ oxidative addition in iodomethane to form fac-PtIMe2{(pz)2(C3H2N2)CH-N,N’,C} (13).