Experimental observation of spin delocalisation onto the aryl-alkynyl ligand in the complexes [Mo(CCAr)(Ph2PCH2CH2PPh2)(η-C7H7)]+ (Ar = C6H5, C6H4-4-F; C7H7 = cycloheptatrienyl): an EPR and ENDOR investigation

Abstract

The paramagnetic aryl-alkynyl complexes [Mo(CCAr)(dppe)(η-C₇H₇)]⁺ (dppe = Ph₂PCH₂CH₂PPh₂; Ar = C₆H₅, [1]⁺; C₆D₅, [2]⁺; C₆H₄-4-F, [3]⁺; C₆H₄-4-Me, [5]⁺) and [Mo(CCBu^t)(dppe)(η-C₇H₇)]⁺ [4]⁺, have been investigated in a combined EPR and ENDOR study. Direct experimental evidence for the delocalisation of unpaired spin density over the framework of an aryl-alkynyl ligand has been obtained. The X-band solution EPR spectrum of the 4-fluoro derivative, [3]⁺, exhibits resolved hyperfine coupling to the remote para position of the aryl group [a_iso(¹⁹F) = 4.5 MHz, (1.6 G)] in addition to couplings attributable to ^95/97Mo, ³¹P and ¹H of the C₇H₇ ring. A full analysis of the ¹H ENDOR spectra is restricted by the low g anisotropy of the system which prevents the use of orientation selection. However, inter-comparison of the ¹H cw-ENDOR frozen solution spectra of [1]⁺, [2]⁺, [4]⁺ and [5]⁺, combined with spectral simulation informed by calculated values derived from DFT investigations, has facilitated estimation of the experimental a_iso(¹H) hyperfine couplings of [1]⁺ including the ortho, ±3.7 MHz (±1.3 G) and para, ±3.9 MHz (±1.4 G) positions of the C₆H₅ substituent of the aryl-alkynyl ligand.