Synthesis of Singly and Doubly Bridged ansa-Zirconocene Hydrides. Formation of an Unusual Mixed Valence Trimeric Hydride by Reaction of H₂ with {(Me₂Si)₂(η⁵-C₅H₃)₂}Zr(CH₃)₂ and Generation of a Dinitrogen Complex by Reaction of N₂ 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 2 Si(η 5 -C 5 H 3 -3-CMe 3 ) 2 }ZrMe 2 affords the isomeric dimeric dihydrides [{ meso -Me 2 Si(η 5 -C 5 H 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 2 Si(η 5 -C 5 H 2 -2-SiMe 3 -4-CMe 3 ) 2 ZrMe 2 (BpZrMe 2 ) reacts with dihydrogen, affording the first example of a monomeric ansa -zirconocene dihydride, { rac -Me 2 Si(η 5 -C 5 H 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 2 Si) 2 (η 5 -C 5 H 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 2 Si) 2 (η 5 -C 5 H-3,5-(CHMe 2 ) 2 )(η 5 -C 5 H 2 -4-CHMeEt}ZrH] 2 (μ 2 -H) 2 ([ sec -BuThpZrH] 2 (μ 2 -H) 2 ) and [(Me 2 Si) 2 (η 5 -C 5 H-2,4-(CHMe 2 ) 2 )(η 5 -C 5 H 2 -4-CHMe 2 )ZrH] 2 (μ 2 -H) 2 ([ i PrThpZrH] 2 (μ 2 -H) 2 ) have been prepared and shown to be robust dimers in solution.