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 alzsa-zirconocene dihydride complexes has been prepared from hydrogenation of the corresponding dimethyl complexes. For the singly [SiMe2]-bridged species, the hydrogenation reaction is facile at 25 degreesC, whereas for the doubly [SiMe2]-bridged complexes hydrogenation occurs over the course of days at 87 degreesC, Hydrogenation of {meso-Me2Si(eta (5)-C5H3-3-CMe3)(2)}ZrMe2 affords the isomeric dimeric dihydrides [{meso-Me2Si(eta (5)-C5H3-3-CMe3)(2)}ZrH](2) (2)(mu (2)-H)(2), one of which has been characterized by X-ray diffraction, The raceme isomer of Me2Si(eta (5)-C5H2-2-SiMe3-4-CMe3)(2)ZrMe2 (BpZrMe(2)) reacts with dihydrogen, affording the first example of a monomeric ansa-zirconocene dihydride, {racMe(2)Si(eta (5)-C5H2-2-SiMe3-4-CMe3)(2)}ZrH2 (rac-BpZrH(2)). In the presence of dinitrogen, racBpZrH(2) undergoes thermal reductive elimination of Ha and reaction with N-2, yielding the dinitrogen complex BpZr(mu (2),eta (2)-eta (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) Angstrom. The doubly [SiMe2]-bridged zirconium dimethyl complex {(Me2Si)(2)(eta (5)-C5H3)(2)}ZrMe2 (RpZrMe(2)) undergoes hydrogenation under forcing conditions, affording the mixed valent (Zr(IV)/Zr(IV)/Zr(III)) hydride trimer [RpZr](3)(mu (3)-H)(2)(mu (2)-H)(3), which has been characterized by X-ray diffraction. More substituted doubly [SiMe2]-bridged zirconocene dihydrides such as [{(Me2Si)(2)(eta (5)-C5H-3,5(CHMe2)(2))(eta (5)-C5H2-4-CHMeEt}ZrH](2)-(mu (2)-H)(2) ([sec-BuThpZrH](2)(mu (2)-H)(2)) and [(Me2Si)(2)(eta (5)-C5H-2,4-(CHMe2)(2))(eta (5)-C5H2-4-CHMe2)ZrH](2) (mu (2)-H)(2) ([(i)PrThpZrH](2)(mu (2)-H)(2)) have been prepared and shown to be robust dimers in solution.