Oxygen Atom Transfer from Pyridylpyridazine Chelated Oxidorhenium(V) Complex to Diphosphines: Twin Isomerization, Effects of Diphosphine Spacer Length and Metal Oxidation State

Abstract

The bimolecular reaction of Re V OCl 3 (dppz), 1, (where dppz is 3,6-di(2-pyridyl) pyridazine) with excess diphosphine Ph 2 P(CH 2 ) x PPh 2 (x=1-4) has furnished Re III (OPPh 2 (CH 2 ) x PPh 2 )Cl 3 (dppz), 2 with a dangling phosphine function which is spontaneously converted in solution to Re III (PPh 2 (CH 2 ) x P(O)Ph 2 )Cl 3 (dppz), 3. The complex 2 (x=1) on reaction with excess 1 affords mononuclear Re III (OPPh 2 (CH 2 )P(O)Ph 2 )Cl 3 (dppz), 4 while for x= 2-4 binuclear complex (dppz)Cl 3 Re III (OPPh 2 (CH 2 ) x PPh 2 O)Re III Cl 3 (dppz), 5 was obtained. Rate studies of the twin isomerization (linkage-cum-geometrical) reaction 23 revealed that the reaction is intramolecular in nature. It is initiated by the nucleophilic attack of the metal by the dangling phosphine function. The process slows down nearly exponentially as the diphosphine spacer length x increases. The oxidized complex [Re IV (OPPh 2 (CH 2 )PPh 2 )Cl 3 (dppz)]ReO 4 , 6 does not isomerize.137 keV(7%)) and 188 Re (17 ht 1/2 , 2.11 MeV  -, 155 keV(15%)) emit particles of therapeutically useful energies 1-3 . Second, the rhenium complexes are structural models of RESEARCH ARTICLE 705 Chem Sci Trans., 2015, 4(3), 704-715 their less easily handled technetium (99mTc) analogues that find use in nuclear medicine, particularly as heart imaging agents 1 . Third, many such rhenium compounds catalyze oxygen atom transfer (OAT) reactions, as recently reviewed 3-5 . Our interest in the Re(III) complexes stems from the expectation that d 4 Re(III) complexes will be kinetically more inert than d 2 Re(V) complexes and, thus, may be useful for radio therapeutic applications 1-5 . Recently we reported oxido chelate Re V OCl 3 (dppz), 1, where dppz is 3,6-di(2-pyridyl) pyridazine) which underwent facile bimolecular oxygen atom transfer reaction with triphenyl phosphine furnishing the corresponding phosphine oxide complex Re III (OPPh 3 )Cl 3 (dppz) as shown in Eq. 1 6 .