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Synthesis of Singly and Doubly Bridged ansa-Zirconocene Hydrides. Formation of an Unusual Mixed Valence Trimeric Hydride by Reaction of H2 with {(Me2Si)25-C5H3)2}Zr(CH3)2 and Generation of a Dinitrogen Complex by Reaction of N2 with a Zirconocene Dihydride

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journal contribution
posted on 2001-01-11, 00:00 authored by Paul J. Chirik, Lawrence M. Henling, John E. Bercaw
A series of singly and doubly bridged ansa-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 °C, whereas for the doubly [SiMe2]-bridged complexes hydrogenation occurs over the course of days at 87 °C. Hydrogenation of {meso-Me2Si(η5-C5H3-3-CMe3)2}ZrMe2 affords the isomeric dimeric dihydrides [{meso-Me2Si(η5-C5H3-3-CMe3)2}ZrH]22-H)2, one of which has been characterized by X-ray diffraction. The racemo isomer of Me2Si(η5-C5H2-2-SiMe3-4-CMe3)2ZrMe2 (BpZrMe2) reacts with dihydrogen, affording the first example of a monomeric ansa-zirconocene dihydride, {rac-Me2Si(η5-C5H2-2-SiMe3-4-CMe3)2}ZrH2 (rac-BpZrH2). In the presence of dinitrogen, rac-BpZrH2 undergoes thermal reductive elimination of H2 and reaction with N2, yielding the dinitrogen complex BpZr(μ222-N2)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 [SiMe2]-bridged zirconium dimethyl complex {(Me2Si)25-C5H3)2}ZrMe2 (RpZrMe2) undergoes hydrogenation under forcing conditions, affording the mixed valent (Zr(IV)/Zr(IV)/Zr(III)) hydride trimer [RpZr]33-H)22-H)3, which has been characterized by X-ray diffraction. More substituted doubly [SiMe2]-bridged zirconocene dihydrides such as [{(Me2Si)25-C5H-3,5-(CHMe2)2)(η5-C5H2-4-CHMeEt}ZrH]22-H)2 ([sec-BuThpZrH]22-H)2) and [(Me2Si)25-C5H-2,4-(CHMe2)2)(η5-C5H2-4-CHMe2)ZrH]22-H)2 ([iPrThpZrH]22-H)2) have been prepared and shown to be robust dimers in solution.

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