Synthesis and Catalytic Application of Heterobimetallic Complexes

Tris(phosphinolamide) early/late heterobimetallic Zr/Co complexes, ClZr(R&feet;NPR 2)3CoI [R&feet; = iPr, R = Ph (2.1), R&feet; = 2,4,6-trimethylphenyl, R = iPr (2.2), R&feet; = R = iPr (2.3)], have been utilized as catalysts for the cross-coupling of alkyl halides with n-octylmagnesium bromide. While yields are consistently higher for alkyl bromide substrates, it is found that these unusual heterobimetallic complexes are also active towards more challenging alkyl chloride substrates. This is particularly interesting in light of the fact that monometallic cobalt complexes are inert towards these substrates, suggesting that Zr also plays a role in catalysis. Radical trapping studies suggest that a one-electron transfer radical oxidative addition pathway is operative.;The tris(phosphinoamide)-linked heterobimetallic Co/Zr complex (THF)Zr(MesNP iPr2)3CoN2 (3.1) has been investigated as a catalyst for the hydrosilylation of ketones with PhSiH3. Catalytic activity superior to monometallic Co or Zr analogues has been observed, demonstrating the importance of cooperative reactivity between Co and Zr. Upon examining stoichiometric reactions, complex 3.1 was found to be unreactive toward PhSiH3, implying that the mechanism diverges from the typical Chalk--Harrod-type hydrosilylation pathway. In contrast, 3.1 reacts readily with ketones, and in the case of benzophenone, a radical coupling product [(Ph2CO)Zr(MesNP iPr2)3CoN2]2 (3.3) was isolated, implying the intermediacy of a Zr-bound ketyl radical fragment. A radical-based hydrosilylation mechanism is proposed involving hydrogen atom transferring from PhSiH3 to the Zr-bound ketyl-radical.;A phosphinoamide ligand, [iPr2PNAr] - (Ar = 3,5-dimethylphenyl), has been introduced to support the Zr/Co system. Heterobimetallic Co/Zr complex (THF)Zr(ArNPiPr 2)3CoN2 (4.4) has been investigated towards C-X (X=halides) activation. It shows very different reactivity compared to (THF)Zr(MesNPiPr2)3CoN 2 (3.1). Further, complex 4.4 shows superior performance to complex 3.1 on catalytic hydrosilylation of ketones. In addition, ICo(iPr2PNAr) 2(&mgr;-I)Zr(&eegr;2-iPr 2PNAr) shows decreased yield compared to ClZr(MesNPi Pr2)3CoI as a catalyst for Kumada coupling reactions. The results show that the steric and electronic effects at the Zr center could significantly affect the reactivity of Zr/Co complexes.;Different combinations of Lewis acidic and Lewis basic metals have been investigated. Al/Co complexes supported by tris(phosphinoamide) ligand could not be made. Different ligand systems have also been investigated. The short phosphine arms of PNNP ligand could not support monomeric Al/Pd complexes. A tetradentate ligand with longer phosphine arms is able to support a series of Pd/Al complexes. The interaction between Pd and Al centers is very strong. A Pt/Alane complex featuring strong d-p dative interacion was also synthesized. Last, a carbine analogue Sn pincer ligand was synthesized. Further coordination chemistry of the Sn ligand should be investigated.