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

The paramagnetic aryl-alkynyl complexes [Mo(C CAr)(dppe)(eta-C7H7)](+) (dppe = Ph2PCH2CH2PPh2; Ar = C6H5, [1](+); C6D5, [2](+); C6H4-4-F, [3](+); C6H4-4-Me, [5](+)) and [Mo(C CBut)(dppe)(eta-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)(F-19) = 4.5 MHz, (1.6 G)] in addition to couplings attributable to Mo-95/97, P-31 and H-1 of the C7H7 ring. A full analysis of the H-1 ENDOR spectra is restricted by the low g anisotropy of the system which prevents the use of orientation selection. However, inter-comparison of the H-1 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)(H-1) 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.