Addition of Terminal Alkynes to a Molecular Titanium–Zinc Nitride

Abstract

Dedicated to Professor Gerhard Erker on the occasion of his 60th birthdayTransition-metal nitrido complexes have been studied extensively in the past few decades. [1, 2] While nitrido complexes of Group 6-8 metals usually bear the nitrido ligands as a terminal functionality, M N, [3] the analogous derivatives of early transition metals exhibit polynuclear structures, with m nnitrido ligands bridging two or more metal centers. [2,4,5] The reactivity of terminal nitrido ligands has received increasing attention because of their potential to show either nucleophilic or electrophilic character. [1,3,6] The net result of the latter is usually a two-electron reduction of the metal center and a reduced bond order between the metal and the nitrogen atom [Eq. (1)]. Recently a similar behavior has been pro-posed for the nucleophilic attack of a phosphine to a bridging dinitride species; in that case the reduction leads to a complex with a metal-metal bond.[7]Over the last few years we have been studying the reactivity of the trinuclear imido-nitrido titanium derivative [{Ti(h 5 -C 5 Me 5 )(m-NH)} 3 (m 3 -N)] [8] (1) towards metal complexes and have reported the coordination of 1 to these metals through the imido groups to give the cube-type adducts [1·ML n ].[9] The elimination of organic molecules and the formation of nitrido groups that bridge the metal centers occur if the NÀH bonds of the organometallic ligand 1 are activated by metal-imido, metal-amido, or metal-alkyl linkages.[10] However, none of those processes involves the triply bridging apical nitrido ligand of 1. Herein we present the first example of a reaction at this nitrido ligand, which leads to an alkynylimido ligand by C À N bond formation. We have studied this "apparent" nucleophilic attack of an acetylide at the apical nitrido ligand by density functional calculations and propose a plausible reaction mechanism.Our synthetic route is outlined in Scheme 1. Treatment of the amido zinc derivative 2 [11] with one equivalent of terminal alkyne RCCH in toluene gives the expected zinc acetylides [{(RCC)Zn}(m 3 -N)(m 3 -NH) 2 {Ti(h 5 -C 5 Me 5 )} 3 (m 3 -N)], with R = SiMe 3 (3), CMe 3 (4), or Ph (5), as brown or red solids in good yields (71-87 %). Because of the light sensitivity of 2 in solution, the reactions were performed in amber-stained glassware. Complexes 3 and 4 were obtained at room temperature, while 5 had to be prepared at 90 8C to obtain a pure sample. Reaction of 2 with two equivalents of Me 3 SiCCH led to a complex which was shown by X-ray diffraction to be the zinc acetylide [{(RCC)Zn}(m 3 -NH) 3 {Ti-(h 5 -C 5 Me 5 )} 3 (m 3 -NCCR)], with R = SiMe 3 (6). The analogous reaction with PhCCH gave the complex with R = Ph (7). Both 6 and 7 were obtained as black solids in good yields (81 and 91 %, respectively). In contrast, the reaction of 2 with more than two equivalents of tert-butylacetylene at a variety of temperatures only produced a mixture of 4 and the unreacted alkyne. [D 6 ]Benzene solutions of 6 are stable at different temperatures, wh...