Nuclear Magnetic Resonance Spectra of Titanocene Sandwich Compounds

Abstract

Proton chemical shifts of cyclopentadienyl ring protons in a series of substituted bis(cyclopentadienyl)titanium compounds have been correlated with electronegativities of the ligands on titanium.A similar correlation of ring proton shifts with ligand electronegativities was found for a series of methyl-substituted titanocene halides and titanocene dihalides, and a reverse trend was noted when the chemical shift of the methyl protons was considered in the methyl-substituted series. In a series of substituted diaryltitanocenes the average chemical shift of ortho and meta protons relative to the chemical shift of a reference (bis(cyclopentadienyl)diphenyltitanium) as well as the chemical shifts of the cyclopentadienyl ring protons give a linear correlation with Taft's inductive and resonance parameters and a satisfactory Hammett line.

ansa-Metallocene derivatives XX. An assignment of 1H- and 13C-NMR signals in ansa-metallocene complexes with 16-electron and 18-electron configuration

“…The mean values of a-H and f3-H shifts for each of the dihalogenide complexes la-d reveal the same trend, i.e. the greatest shielding for the difluoro and the greatest deshielding for the diiodo species, as the (rotationally averaged) ring-H shifts measured for the corresponding un bridged titanocene dihalides, (CsHshTiX2 [16,21,22]. The apparent H(a,f3) coupling constants of 2.5-2.7 Hz are comparable with those observed in unbridged substituted titanocene derivatives [23].…”

“…1) must evidently be connected with changes in the electronic structure of these compounds. The high-field shift of the cyclopentadienyl protons in the related 16-electron series (CSH5hTiX2 from X = r to X = F-, which is opposite to that expected from the associated electronegativity changes, has been ascribed to an inGreasing tendency of the lighter halogens to transfer electrons to the metal centre by 'lTp.d back donation [16,21,22]. This view is not supported, however, by a series of Extended HUckel Molecular Orbital calculations that we conducted on complexes la, Ib and Id *, which yield positive charges of 2.28, 1.66 and 0.56 units on the Ti centre of the difluoro, dichloro and diiodo derivatives, respectively, wholly in accord with the expected electronegativity effects.…”

[1]Metallocenophane: 1H-NMR-Untersuchungen / [1]Metallocenophanes: 1H NMR Investigations

“…führen sollte [10]. Dies wurde durch die Bildung einer p.T->djr-Rückbindung zwischen Halogen und Zentralatom interpretiert [5], die beim F am stärksten [12][13][14] des Winkels <p von 120-150° am Zentralatom M dadurch aufrechterhalten wird, daß der Winkel co am Fünfringzentrum zu kleineren Werten als 90° deformiert wird (vgl. Abb.…”

“…Einfache Metallocene, wie z.B. (CsHs^TiCl^ zeigen in ihrem 1 H-NMR-Spektrum wegen der freien Rotation der Cyclopentadienylringe für alle zehn Protonen ein scharfes Singulett [5]. Bei [l]Metallocenoprotonen zwei triplettartige Multipletts vom AA'XX'-Kopplungstyp beobachtet [1], Abb.…”

“…Zur Deutung könnte man eine analoge sterisehe Wirkung des Halogens, wie Abb. 2b zeigt, heranziehen.Gegen diese Interpretation spricht jedoch die Tatsache, daß wie bei den entsprechenden Metallocenen wahrscheinlich gemacht wurde[5], die Bildungsmöglichkeit einer pn-+dji-Bindung zwischen Halogen und Zentralatom berücksichtigt werden muß. Außerdem erwartet man bei Berücksichtigung von räumlichen Einflüssen ein Anwachsen von Ar mit steigender Größe des Zentralatoms M, was den Befunden widerspricht (Tab.…”