Substituent effects on 1H and 13C NMR chemical shifts in titanocene benzoates

Abstract

¹ H and ¹³C NMR chemical shifts of meta and para-substituted benzoato titanocene complexes of the type (C₅H₅)₂Ti(O₂CC₆H₄X)₂ have been measured. Data analysis of the phenyl carbons were performed using the dual substituent parameter approach (para-carbons) or triple parameter approach (ipso-, ortho-, and meta-carbons), showing good to excellent correlations. The signs and magnitudes of the ρ-values agreed very well with literature results, proving the accuracy of the data set and the consistency of the procedure, as well as pointing to common mechanistic origins for ortho, meta and para substituent chemical shifts (SCS) in substituted benzoato titanocenes and monosubstituted benzenes. A dual parameter approach was then used to analyse the carbons of the carbonyl group and the cyclopentadienyl rings. A reverse SCS effect was observed for the carboxylato group and an extremely long-range interaction between substituent X and the cyclopentadienyl carbon atoms was observed. The latter ρ-values follow normal field and mesomeric influences. Similar results are obtained from proton shifts. The bulky group (C₅H₅)₂(XC₆H₄CO₂)TiCO_2– was found to be a very weak inductive acceptor (σ_F= 0.035) and a moderate resonance acceptor (σ⁰_R= 0.142).