Organometallic Complexes for Nonlinear Optics. 3.¹Molecular Quadratic Hyperpolarizabilities of Ene-, Imine-, and Azo-Linked Ruthenium σ-Acetylides:  X-ray Crystal Structure of Ru((E)-4,4‘-C⋮CC₆H₄CHCHC₆H₄NO₂)(PPh₃)₂(η-C₅H₅)

The series of complexes Ru(( E )-4,4‘-C⋮CC 6 H 4 X CHC 6 H 4 NO 2 )(PR 3 ) 2 (η-C 5 H 5 ) (X = CH, R = Ph, 11a; X = CH, R = Me, 11b; X = N, R = Ph, 12a; X = N, R = Me, 12b ) has been synthesized by reaction of RuCl(PR 3 ) 2 (η-C 5 H 5 ) with ( E )-4,4‘-HC⋮CC 6 H 4 X CHC 6 H 4 NO 2 and deprotonation of the intermediate vinylidene complex. Complex 11a has been structurally characterized; it is the first example of a donor-acceptor organometallic “extended” chromophore bearing the prototypical acceptor −NO 2 to be crystallographically studied. Molecular quadratic hyperpolarizabilities at 1.9 μm were evaluated computationally for the complexes above and imine- and azo-linked analogues by employing ZINDO with crystallographically obtained atomic coordinates. The results are consistent with a substantial increase in quadratic nonlinearity for (i) chain lengthening of the organometallic chromophore (replacing 4-C⋮CC 6 H 4 NO 2 by ( E )-4,4‘-C⋮CC 6 H 4 CH CHC 6 H 4 NO 2 ) and (ii) an azo linkage compared with an ene linkage (replacing ( E )-4,4‘-C⋮CC 6 H 4 CH CHC 6 H 4 NO 2 by ( E )-4,4‘-C⋮CC 6 H 4 N NC 6 H 4 NO 2 ). Little variation in computed response was found upon substituting an imine linkage for an ene linkage in the organometallic chromophore (replacing ( E )-4,4‘-C⋮CC 6 H 4 CH CHC 6 H 4 NO 2 by ( E )-4,4‘-C⋮CC 6 H 4 N CHC 6 H 4 NO 2 or ( E )-4,4‘-C⋮CC 6 H 4 CH NC 6 H 4 NO 2 ). Molecular quadratic optical nonlinearities were determined experimentally for 11a, 12a, and Ru(C⋮CC 6 H 4 NO 2 -4)(PR 3 ) 2 (η-C 5 H 5 ) (R = Ph, Me) by electric-field-induced second-harmonic generation (EFISH; 11a and 12a only) and hyper-Rayleigh scattering (HRS) techniques. EFISH-derived μβ 1064 values for 11a (9700 × 10 - 48 cm 5 esu - 1 ) and 12a (5800 × 10 - 48 cm 5 esu - 1 ) are large compared to those for other organometallic complexes. Resonance-enhanced quadratic nonlinearities at 1.06 μm from HRS are large (1455 × 10 - 30 cm 5 esu - 1, 11a; 840 × 10 - 30 cm 5 esu - 1, 12a ). Two-level-corrected values confirm a substantial increase in quadratic nonlinearity for chain lengthening but suggest a significant decrease in nonlinearity on replacing an ene linkage by an imine linkage; the latter is contrary to the ZINDO result, and the reasons for this are discussed.