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...

Cleavage of Dinitrogen from Forming Gas by a Titanium Molecular System under Ambient Conditions

“…In a fashion similar to that transformation, the reaction of 2 with NH 3 ⋅ BH 3 in [D 8 ]tetrahydrofuran at 85 °C was monitored by 11 B NMR spectroscopy and the spectra showed one triplet resonance [ δ =−10.5 ppm; 1 J (B,H)=101.4 Hz] assigned to cyclotriborazane (CTB; [NH 2 BH 2 ] 3 ) and minor resonance signals for borazine and polyborazylene . However, the diamagnetic nature of 5 is consistent with the hydrogenation of the two μ 3 ‐nitrido ligands of 2 with concomitant two‐electron reduction of the Ti 4 core similar to that observed in the electronic structure of [(RCC)Zn{(μ 3 ‐NH) 3 Ti 3 Cp* 3 (μ 3 ‐NCCR)}] . The molecular structure of 5 shows a [Ti 4 (μ 3 ‐E) 4 ] cube‐type core (Supporting Information Figure S3) similar to those found for complexes 1 , 3 and 4 , with averaged Ti−E (E=C or N) and Ti−Ti distances of 2.040(4) and 2.809(1) Å, respectively .…”

“…[19] However, the diamagneticn ature of 5 is consistentw ith the hydrogenation of the two m 3 -nitrido ligands of 2 with concomitantt wo-electron reduction of the Ti 4 core similart ot hat observed in the electronic structure of [(RCC)Zn{(m 3 -NH) 3 Ti 3 Cp* 3 (m 3 -NCCR)}]. [20] The molecular structure of 5 shows a[ Ti 4 (m 3 -E) 4 ]c ube-type core (Supporting Information Figure S3) similar to those found for complexes 1, 3 and 4,w ith averagedT i ÀE( E = Co rN )a nd TiÀTi distances of 2.040(4) and 2.809(1) ,r espectively. [10] The 1 HNMR spectrum of 5 in [D 1 ]chloroform shows two sharp singlets fort he h 5 -C 5 Me 5 groups in a1 :1 ratio, one singlet at d = 15.68 ppm for two methylidyne ligands and ab road resonance signal at d = 12.72 ppm for the NH groups in accordance with a C 2v symmetry in solution.…”

Discovering the chemical reactivity of the molecular oxonitride [{Ti(η5-C5Me5)(μ-O)}3(μ3-N)]

“…Geometry optimizations were carried out under C 3v symmetry restrictions. Note that the energy values of the molecular orbitals are slightly different to those previously reported [4,11,36] because the DFT functional and basis set are different. …”

Reactivity with Electrophiles of Imido Groups Supported on Trinuclear Titanium Systems

“…14 This nitrido ligand is quite chemically unreactive, although we have reported the "apparent" nucleophilic attack of an acetylide group [C≡CR]at this site to yield alkynylimido 3-NCCR ligands with a two-electron reduction of the Ti3 core. 15 Density functional theory (DFT) calculations showed that this reaction involves the formation of an alkynyl titanium intermediate followed by migration of the alkynyl group to the apical nitrido ligand. We were interested in studying the reactivity of 1 toward electrophiles and here we report on the reaction with one equivalent of ROTf to generate polynuclear complexes by selective functionalization of the imido groups.…”