Synthesis of Singly and Doubly Bridgedansa-Zirconocene Hydrides. Formation of an Unusual Mixed Valence Trimeric Hydride by Reaction of H_2 with {(Me_2Si)_2(η^5-C_5H_3)_2}Zr(CH_3)_2and Generation of a Dinitrogen Complex by Reaction of N_2 with a Zirconocene Dihydride

A series of singly and doubly bridged ansa-zirconocene dihydride complexes has been prepared from hydrogenation of the corresponding dimethyl complexes. For the singly [SiMe_2]-bridged species, the hydrogenation reaction is facile at 25 °C, whereas for the doubly [SiMe_2]-bridged complexes hydrogenation occurs over the course of days at 87 °C. Hydrogenation of {meso-Me_2Si(η^5-C_5H_3-3-CMe_3)_2}ZrMe_2 affords the isomeric dimeric dihydrides [{meso-Me_2Si(η^5-C_5H_3-3-CMe_3)_2}ZrH]_2(μ_2-H)_2, one of which has been characterized by X-ray diffraction. The racemo isomer of Me_2Si(η^5-C_5H_2-2-SiMe_3-4-CMe_3)_2ZrMe_2 (BpZrMe_2) reacts with dihydrogen, affording the first example of a monomeric ansa-zirconocene dihydride, {rac-Me_2Si(η^5-C_5H_2-2-SiMe_3-4-CMe_3)_2}ZrH_2 (rac-BpZrH_2). In the presence of dinitrogen, rac-BpZrH_2 undergoes thermal reductive elimination of H_2 and reaction with N_2, yielding the dinitrogen complex BpZr(μ_2,η^2,η^2-N_2)ZrBp, for which the dinitrogen ligand is coordinated in a side-on fashion to both zirconiums with a N−N bond distance of 1.241(3) Å. The doubly [SiMe_2]-bridged zirconium dimethyl complex {(Me_2Si)_2(η^5-C_5H_3)_2}ZrMe_2 (RpZrMe_2) undergoes hydrogenation under forcing conditions, affording the mixed valent (Zr(IV)/Zr(IV)/Zr(III)) hydride trimer [RpZr]_3(μ_3-H)_2(μ_2-H)_3, which has been characterized by X-ray diffraction. More substituted doubly [SiMe_2]-bridged zirconocene dihydrides such as [{(Me_2Si)_2(η^5-C_5H-3,5-(CHMe_2)_2)(η^5-C_5H_2-4-CHMeEt}ZrH]_2(μ_2-H)_2 ([sec-BuThpZrH]_2(μ_2-H)_2) and [(Me_2Si)_2(η^5-C_5H-2,4-(CHMe_2)_2)(η^5-C_5H_2-4-CHMe_2)ZrH]_2(μ_2-H)_2 ([^iPrThpZrH]_2(μ_2-H)_2) have been prepared and shown to be robust dimers in solution