Modifying Alkylzinc Reactivity with 2,2′-Dipyridylamide: Activation of tBu=Zn Bonds for para-Alkylation of Benzophenone**

Many of the recent eye-catching advances in zincate executed metallation chemistry have centered on the secondary monoamide 2,2,6,6-tetramethylpiperidide (TMP). Developed by Kondo, Uchiyama, and co-workers, the original TMP zincate reagent [LiZn(TMP)tBu2] is an effective base for both aromatic and heteroaromatic substrates. Knochel and coworkers have since compiled a library of new TMP-based metalating agents, including the zinc pivalate [(TMP)Zn(OPiv)·LiCl] (OPiv= pivalate), which like [LiZn(TMP)tBu2], displays strong deprotonating power and exceptional functional-group tolerance but has the added advantage that its arylzincated derivatives boast prolonged air stability. The area of zincate metalation chemistry that is best structurally defined is that involving the sodium TMP reagent [(TMEDA)Na(TMP)(tBu)Zn(tBu)] (1; TMEDA= N,N,N’,N’-tetramethylethylenediamine). This reagent, which exhibits enhanced reactivity over the aforementioned lithium zincates, and many of its arylzincated derivatives adopt similar structures, which are designed on an architectural template of a Na–anion–Zn–anion ring carrying terminal ligands, namely a neutral donor and an anion on Na and Zn, respectively (Figure 1). To break this template and develop new structural motifs that could stimulate new reactivity we have investigated replacing monofunctional TMP with multifunctional 2,2’-dipyridylamide [dpa, (2-NC5H4)2N]. Offering three potential ligating N sites, this N-bridged bis(N heterocycle) finds utility in a diverse range of applications including medicine, catalysis, photoluminescence, and supramolecular chemistry. Herein we report a remarkable set of novel structures incorporating dpa into neutral zinc, sodium zincate, and potassium zincate modifications containing tert-butyl ligands. Preliminary reactivity studies reveal that the mixed sodium-zinc dpa complexes can tert-butylate the ketone benzophenone at the para (1,6-addition) position, which is challenging, and hint that the reaction can be made substoichiometric in the sodium component. The neutral, heteroleptic zinc complex [{(dpa)Zn(tBu)}2] (2) was synthesized straightforwardly through the 1:1 stoichiometric combination of tBu2Zn and the amine dpa(H) in a hexane solution. NMR studies of the filtrate revealed the absolute yield was higher than the 70% yield of the isolated crystals. Possessing an attractive hour-glass-shaped core, the centrosymmetric molecular structure of 2 (Scheme 1) is dimeric with the distorted tetrahedral zinc atom bonded to the deprotonated N of one amido unit, which occupies the dipyridyl pocket of the other and completes its coordination with a terminal tBu ligand. In the hour-glass description (see the Supporting Information), each bulb comprises a puckered, six-atom (NCNCNZn) ring with Zn situated 1.0991(3) from the nearest NCNCN plane and 1.7143(3) from the second NCNCN plane. This motif bears a strong resemblance to that of the isoelectronic neutral zinc dimer [{MeZnC(H)Py2}2](Py=2-pyridyl). [8] To our knowledge, 2 represents the first crystallographically characterized Zn/dpa complex showing a Zn (bridgehead)N bond, although it has been noted in alkylated dimeric derivatives wherein the mode of dimerization is distinct from that in 2. The anti/anti conformation (i.e., both pyridyl N atoms directed away from the bridgehead N) within 2 also contrasts with the syn/anti conformations found in polymorphs of dpa(H) and the aforementioned alkylated dpa derivatives. A simple transamination reaction between TMP/zincate 1 and dpa(H) in a hexane solution was anticipated, but instead the isolated crystalline product was the disodium monozinc species [(TMEDA)2Na2(m-dpa)2Zn(tBu)2] [3 ; 70% yield based on the dpa(H) stoichiometry]. Formally transamination has occurred but the 2:1 Na/Zn stoichiometry of 3 is inconsistent with the 1:1 stoichiometry of the reaction. Its molecular structure (Figure 2) can be viewed as a cocomplex between a TMEDA-complexed sodium amide dimer and Figure 1. General structural motif observed for heteroleptic ate bases. Here the subordinate metal is zinc.