Tridentate SNS Research Articles

Ethylene Trimerization with Mixed-Donor Ligand (N,P,S) Chromium Complexes: Effect of Ligand Structure on Activity and Selectivity

Methylaluminoxane (MAO)-activated chromium(III) complexes of tridentate PNP and SNS ligands of the form (R2PCH2CH2)2NR‘ (R = alkyl, aryl; R‘ = H, Me, benzyl) and (RSCH2CH2)2NH (R = alkyl) have been prepared and tested for the trimerization of ethylene to 1-hexene. The effect of P or S donor substitution, nitrogen substitution, and chelate ring size has been examined. Sterically compact and basic P or S groups lead to the highest activities and selectivities, while a secondary amine central donor is crucial for high activity, raising the possibility that deprotonation of this group occurs during catalyst formation. Expanding the chelate ring size by introduction of a propyl spacer on one side of the SNS ligand gives EtSC3H6N(H)C2H4SEt, which leads to a trimerization catalyst with lower activity. The scope of S-donor-based ligands for ethylene trimerization was further studied by preparing and testing PSP and SPS tridentates of CrIII. These complexes give rise to active oligomerization systems that give product distributions characteristic of both linear chain growth and selective trimerization.

First Cr(III)-SNS complexes and their use as highly efficient catalysts for the trimerization of ethylene to 1-hexene.

Cr(III) complexes of tridentate SNS ligands have been prepared and evaluated as catalysts for ethylene trimerization, with several giving very high activity and excellent selectivity toward 1-hexene when activated with methylaluminoxane. The new complexes illustrate the potential of sulfur-based ligands on early transition metals for catalysis.

Study on the formation of mixed ligand oxorhenium and oxotechnetium complexes (SNS/S combination)

Theoretically, several complexes can be formed during the reaction between the tridentate aminedithiol ligand EtN(CH 2CH 2SH) 2, L1H 2, the monodentate thiol p-ClC 6H 4SH, L2H, and the Re VOCl 3(PPh 3) 2. Three main possibilities are: (i) neutral mixed ligand complexes ReOL1L2, the syn isomer, complex 1 and the anti isomer, complex 2; (ii) binuclear complex of the tridentate ligand, (ReO) 2(L1) 3, complex 3 and (iii) anionic complex of the monothiol [ReO(L2) 4] −, complex 4. When a mixture of L1H 2/L2H, 1/1 ratio, is applied, the major product of the reaction is the syn isomer 1. A small amount of the anti isomer 2 is also isolated (yield <2%) while none of the complexes 3 and 4 are formed under the above reaction conditions. The oxorhenium complexes 3 and 4 have been synthesized by the reaction of L1H 2 or L2H respectively with the precursor ReOCl 3(PPh 3) 2. The crystal structures of 1, 3, and 4 are determined by X-ray crystallography. The corresponding 99mTc complexes have been prepared by exchange reaction using 99mTc-glucoheptonate as precursor. Similarly the major reaction product is the syn isomer, complex 1′, while none of the other complexes are formed during the reaction at tracer level. The above studies demonstrate that simultaneous action of a tridentate SNS ligand and a monodentate thiol in equimolar quantities on Re VOCl 3(PPh 3) 2 or 99mTc-glucoheptonate leads to a single rhenium or technetium-99m species, the syn MOL1L2.

Tridentate ligands containing the SNS donor atom set as a novel backbone for the development of technetium brain-imaging agents.

In developing 99mTc complexes as potential brain-imaging agents, we investigated the coordination chemistry of ligands containing sulfur and nitrogen donor atoms with the general formula R-CH2CH2N(CH2CH2SH)2 (R = C2H5S, (C2H5)2N). These ligands act as tridentate SNS chelates to the TcO3+ core, leaving open one coordination site cis to the oxo group. In reactions with the highly reactive [99TcOCl4]- precursor, this vacancy was occupied by a chlorine atom. When the ligands reacted in the presence of 4-methoxythiophenol, using 99Tc(V)-gluconate as precursor, the vacancy was filled with 4-methoxythiophenol, which acted as coligand. Thus neutral mixed ligand complexes of the general formula [TcO((SCH2CH2)2NCH2CH2R)X], where X = Cl or 4-methoxythiophenol, were synthesized. The complexes were characterized by UV-vis, IR, 1H NMR, crystallographic, and elemental analyses. The crystal structures of 3a (R = C2H5S, X = Cl) and 4b (R = (C2H5)2N, X = 4-methoxythiophenol) demonstrated that the coordination geometry is trigonal bipyramidal with the N1 and Cl or S3 occupying the apical positions and the basal plane defined by the S1 and S2 of the tridentate ligand and the oxo group. The complexes 4a(99mTc) (R = C2H5S, X = 4-methoxythiophenol) and 4b(99mTc) were prepared using 99mTc-glucoheptonate as precursor and were purified by HPLC. Biodistribution in mice showed high initial brain uptake (3.68% and 3.56% dose/organ for 4a(99mTc) and 4b(99m-Tc), respectively). Complex 4b(99mTc) displayed significantly higher brain/blood values and prolonged retention in brain as well. The results suggest that structural modifications based on configurations 4a,b may provide novel 99mTc brain-imaging agents with improved biological characteristics.