Reaction of HC(Me 2 pz) 3 with Mg{N(SiMe 3 ) 2 } 2 gave the zwitterionic amide complex Mg{C-(Me 2 pz) 3 }{N(SiMe 3 ) 2 } (13), containing a "free" pyramidal carbanion and a cationic {Mg-N-(SiMe 3 ) 2 } þ moiety. Reaction of the less bulky amide Mg{N(SiHMe 2 ) 2 } 2 with RC(Me 2 pz) 3 (R=H or D) gave the four-coordinate adducts Mg{κ 2 N-RC(Me 2 pz) 3 }{N(SiHMe 2 ) 2 } 2 (R = H (15) or D (15-d)). A similar complex, Mg{κ 2 N-HC(Me 2 pz) 3 }{N(SiMe 3 ) 2 } 2 (14), was observed by NMR spectroscopy at low temperatures. On warming, 15, 15-d, and 14 eliminated RN(SiR 0 Me 2 ) 2 (R 0 = H or Me) to form Mg{C(Me 2 pz) 3 }{N(SiR 0 Me 2 ) 2 } (13 or 17). The reactions of 15 and 15-d followed first-order kinetics with a primary kinetic isotope effect (k(H)/k(D)) of 1.91(2). Reaction of 13 or 17 with a further equivalent of HC(Me 2 pz) 3 gave the homoleptic sandwich complex Mg{C(Me 2 pz) 3 } 2 (previously reported), containing two free carbanions and a dicationic metal center. The reaction of 17 with an excess of RC(Me 2 pz) 3 (R=H or D) followed pseudo-first-order kinetics with a primary kinetic isotope effect of 1.51(4). The corresponding reactions of Ca{N(SiMe 3 ) 2 } 2 (THF) 2 with 1 or 2 equiv of HC(Me 2 pz) 3 gave Ca{C(Me 2 pz) 3 }{N(SiMe 3 ) 2 }(THF) (18) or Ca{C(Me 2 pz) 3 } 2 (19). In contrast, Zn{N(SiMe 3 ) 2 } 2 was effectively unreactive toward HC(Me 2 pz) 3 . However, reaction of ZnCl 2 with 1 or 2 equiv of Li{C(Me 2 pz) 3 }(THF) afforded Zn{C(Me 2 pz) 3 }Cl (20) or Zn{C(Me 2 pz) 3 } 2 (23), respectively. Subsequent reaction of 20 with NaN(SiMe 3 ) 2 or LiN(SiHMe 2 ) 2 formed Zn{C-(Me 2 pz) 3 }{N(SiRMe 2 ) 2 } (R = Me (21) or H). Cd{N(SiMe 3 ) 2 } 2 reacted with 1 or 2 equiv of HC(Me 2 pz) 3 to form Cd{C(Me 2 pz) 3 }{N(SiMe 3 ) 2 } (24) or Cd{C(Me 2 pz) 3 } 2 (25). Reaction of 13 or 18 with HC(pz) 3 gave the heteroleptic zwitterions M{C(Me 2 pz) 3 }{C(pz) 3 } (M = Mg (26) or Ca (27)), containing κ 3 N-coordinated C(Me 2 pz) 3 and C(pz) 3 ligands. In contrast, reaction of 24 with HC(pz) 3 formed Cd{C(Me 2 pz) 3 }{κ 1 C, κ 2 N-C(pz) 3 } (28), containing two different coordination modes for the tris(pyrazolyl)methanide ligands and a Cd-C(pz) 3 bond.

The first cationic main group tetrahydroborate complexes are reported. [Ca(BH(4))(THF)(5)][BPh(4)] and the charge neutral (Tp((t)Bu,Me))Ca(BH(4))(THF) are initiators for the living ring opening polymerization of rac-lactide, the latter producing PLA with high levels of heterotactic enrichment. These represent a new class of ROP initiators for main group metals.

Reaction of TiCl(2)(Me(2)Calix) with 2 equiv of LiNHNRR' afforded the corresponding terminal hydrazido(2-) complexes Ti(NNRR')(Me(2)Calix) (R = Ph, R' = Ph (1) or Me; R = R' = Me (3)) which were all structurally characterized. The X-ray structure of Ph(2)NNH(2) is reported for comparison. Compound 1 was also prepared from Na(2)[Me(2)Calix] and Ti(NNPh(2))Cl(2)(py)(3). Reaction of ZrCl(2)(Me(2)Calix) with 2 equiv of LiNHNR(2) afforded only the bis(hydrazido(1-)) complexes Zr(NHNR(2))(2)(Me(2)Calix) (R = Ph or Me). Treatment of Ti(NNMe(2))(Me(2)Calix) (3) with MeI gave the zwitterionic hydrazidium species Ti(NNMe(3))(MeCalix) (6) via a net isomerization reaction which was found to be catalytic in MeI. The corresponding reaction of 3 with CD(3)I gave Ti(NNMe(2)CD(3))(MeCalix) (6-d(3)) with concomitant elimination of MeI. Reaction of 3 with 1 equiv of MeOTf gave [Ti(NNMe(3))(Me(2)Calix)][OTf] (7-OTf) which in turn reacted with (n)Bu(4)NI to form 6 and MeI. Addition of PhCHO to 3 gave the mu-oxo dimer [Ti(mu-O)(Me(2)Calix)](2) and benzaldehyde-dimethylhydrazone. Reaction of either 3 or 6 with (t)BuNCO gave the zwitterionic species Ti{(t)BuNC(NNMe(3))O}(MeCalix) (10) which has been crystallographically characterized. Compound 10 is the formal product of insertion of an isocyanate into the Ti=N(alpha) bond of a titanium hydrazide or hydrazidium species (Me(2)Calix or MeCalix = dianion or trianion of the di- or monomethyl ether of p-tert-butyl calix[4]arene, respectively).

The reaction of bis(3,5-dimethylpyrazolyl)methylphenol N(2)O(Ar)H (1) with NaH in THF formed dimeric [Na(kappa(2)-N(2)O(Ar))(THF)](2) (2), which contains a kappa(2)(N,O)-bound bidentate N(2)O(Ar) ligand. The reaction of 1 with Mg(n)Bu(2) gave the four-coordinate monomeric butyl compound Mg(N(2)O(Ar))(n)Bu (3), whereas with (n)BuMgCl, a mixture of products was formed, including the six-coordinate homoleptic species Mg(N(2)O(Ar))(2) (4). The reaction of [Na(kappa(2)-N(2)O(Ar))(THF)](2) with (n)BuMgCl also gave 3, as did the redistribution reaction of Mg(n)Bu(2) with 4. The reaction of 1 with Mg{N(SiRMe(2))(2)}(2) afforded the four-coordinate amide derivatives Mg(N(2)O(Ar)){N(SiRMe(2))(2)} (R = Me (6) or H (7)), together with 4. The reactions of 1 with ZnMe(2) or Zn{N(SiMe(3))(2)}(2) gave the monomeric compounds Zn(N(2)O(Ar))Me (8) and Zn(N(2)O(Ar)){N(SiMe(3))(2)} (9), respectively. The reaction 9 of with HCl formed Zn(N(2)O(Ar))Cl (11), and subsequent addition of LiN(SiHMe(2))(2) to 11 led to Zn(N(2)O(Ar)){N(SiHMe(2))(2)} (12). The reaction of 1 with either Zn{N(SiMe(3))(2)}(2) or 9 gave Zn(N(2)O(Ar))(2). The compounds 2, 3, 4, 6, 8, 9 and 11 were crystallographically characterized. Compound was very active for the ring-opening polymerization (ROP) of epsilon-caprolactone (epsilon-CL) but the process was very poorly controlled as judged by the M(n) and polydispersity index of the polymer. Compounds 3, 8, 9 and 12 gave poor conversions to poly(epsilon-CL) over extended periods. N(2)O(Ar)H = 2,4-di-tert-butyl-6-(bis(3,5-dimethylpyrazolyl)methyl)phenol.

Dicationic, zwitterionic and "conventional" yttrium compounds act as catalysts for the primary or secondary amine-initiated immortal ROP of rac-lactide; amine-terminated, highly heterotactic poly(rac-lactides) with narrow polydispersities and well-controlled molecular weights are prepared in this manner.