Synthesis, structure, and reactivity of zirconium and hafnium imido metalloporphyrins

The zirconium and hafnium porphyrin imido complexes (TTP)M=NAr^iPr [TTP = meso-tetra-p-tolylporphyrinato dianion, M = Zr (1), Hf (2), Ar^iPr = 2,6-diisopropylphenyl] were synthesized from (TTP)MCl₂ and 2 equiv of LiNHAr^iPr. The zirconium imido complex, (TTP)Zr=NAr^iPr, was also obtained from the preformed imido complex Zr(NAr^iPr)Cl₂(THF)₂ and (TTP)Li₂(THF)₂. Treatment of (TTP)HfCl₂ with excess LiNH(p-MeC₆H₄) resulted in the formation of a bis(amido) complex, (TTP)Hf(NH-p-MeC₆H₄)₂ (3), instead of an imido complex. In the presence of excess aniline, 2 formed an equilibrium mixture of bis(amido) compounds, (TTP)Hf(NHPh)(NHAr^iPr) and (TTP)Hf(NHPh)₂. The nucleophilic character of the imido moiety is exhibited by its reaction with ₁BuNCO, producing isolable N,O-bound ureato metallacycles. The kinetic product obtained with zirconium, (TTP)Zr(η²-NAr^iPrC(=N^tBu)O) (4a), isomerized to (TTP)Zr(η²-N^tBuC(=NAr^iPr)O) (4b) in solution. Upon being heated to 80 °C, 4a produced the carbodiimide Ar^iPrN=C=N^tBu and a transient Zr(IV) oxo complex. The analogous hafnium complex (TTP)Hf(η²-NAr^iPrC(=N^tBu)O) (5a) did not eject the carbodiimide upon heating to 110 °C but isomerized to (TTP)Hf(η2-N^tBuC(=NAr^iPr)O) (5b). To support the formulation of 4a and 5a as N,O bound, the complex (TTP)Hf(η²-NAr^iPrC(=NAr^iPr)O) (6) was studied by variable-temperature NMR spectroscopy. The corresponding thio- and selenoureato metallacycles were not isolable in the reaction between 1 and 2 with ^tBuNCS and ^tBuNCSe. Concomitant formation of the metallacycle with decomposition to the carbodiimide, Ar^iPrN=C=N^tBu, reflects the lower C-Ch bond strength in the proposed N,Ch-bound metallacycles. Treatment of 2 with 1,3-diisopropylcarbodiimide resulted in the η²-guanidino complex (TTP)Hf(η²-NAr^iPrC(=NⁱPr)NⁱPr) (7a), which isomerized to the less sterically crowded isomer (TTP)Hf(η²-NⁱPrC(=NAr^iPr)NⁱPr) (7b). Complexes 1, 2, 4a, 4b, and 7a were characterized by X-ray crystallography. The monomeric terminal imido compounds, 1 and 2, are isomorphous: M-N_imido distances of 1.863(2) Å (Zr) and 1.859(2) Å (Hf); M-N_imido-C angles of 172.5(2)° (Zr) and 173.4(2)° (Hf). The structures of the ureato complexes 4a and 4b and the guanidino complex 7a exhibit typical alkoxido and amido bond distances (Zr-N = 2.1096(13) Å (4a), 2.137(3) Å (4b); Zr-O = 2.0677(12) Å (4a), 2.066(3) Å (4b); Hf-N = 2.087(2) Å, 2.151(2) Å (7a)).