Syntheses, Spectroscopic, Electrochemical, and Third-Order Nonlinear Optical Studies of a Hybrid Tris{ruthenium(alkynyl)/(2-phenylpyridine)}iridium Complex

Abstract

The synthesis of fac-[Ir{N,C<inf>1</inf>′-(2,2′-NC<inf>5</inf>H<inf>4</inf>C<inf>6</inf>H<inf>3</inf>-5′-ξC-1-C<inf>6</inf>H<inf>2</inf>-3,5-Et<inf>2</inf>-4-ξCC<inf>6</inf>H<inf>4</inf>-4-ξCH)}<inf>3</inf>] (10), which bears pendant ethynyl groups, and its reaction with [RuCl(dppe)<inf>2</inf>]PF<inf>6</inf> to afford the heterobimetallic complex fac-[Ir{N,C<inf>1</inf>′-(2,2′-NC<inf>5</inf>H<inf>4</inf>C<inf>6</inf>H<inf>3</inf>-5′-ξC-1-C<inf>6</inf>H<inf>2</inf>-3,5-Et<inf>2</inf>-4-ξCC<inf>6</inf>H<inf>4</inf>-4-ξC-trans-[RuCl(dppe)<inf>2</inf>])}<inf>3</inf>] (11) is described. Complex 10 is available from the two-step formation of iodo-functionalized fac-tris[2-(4-iodophenyl)pyridine]iridium(III) (6), followed by ligand-centered palladium-catalyzed coupling and desilylation reactions. Structural studies of tetrakis[2-(4-iodophenyl)pyridine-N,C<inf>1</inf>′](μ-dichloro)diiridium 5, 6, fac-[Ir{N,C<inf>1</inf>′-(2,2′-NC<inf>5</inf>H<inf>4</inf>C<inf>6</inf>H<inf>3</inf>-5′-ξC-1-C<inf>6</inf>H<inf>2</inf>-3,5-Et<inf>2</inf>-4-ξCH)}<inf>3</inf>] (8), and 10 confirm ligand-centered derivatization of the tris(2-phenylpyridine)iridium unit. Electrochemical studies reveal two (5) or one (6-10) Ir-centered oxidations for which the potential is sensitive to functionalization at the phenylpyridine groups but relatively insensitive to more remote derivatization. Compound 11 undergoes sequential Ru-centered and Ir-centered oxidation, with the potential of the latter significantly more positive than that of Ir(N,C′-NC<inf>5</inf>H<inf>4</inf>-2-C<inf>6</inf>H<inf>4</inf>-2)<inf>3</inf>. Ligand-centered π-π∗ transitions characteristic of the Ir(N,C′-NC<inf>5</inf>H<inf>4</inf>-2-C<inf>6</inf>H<inf>4</inf>-2)<inf>3</inf> unit red-shift and gain in intensity following the iodo and alkynyl incorporation. Spectroelectrochemical studies of 6, 7, 9, and 11 reveal the appearance in each case of new low-energy LMCT bands following formal IrIII/IV oxidation preceded, in the case of 11, by the appearance of a low-energy LMCT band associated with the formal RuII/III oxidation process. Emission maxima of 6-10 reveal a red-shift upon alkynyl group introduction and arylalkynyl π-system lengthening; this process is quenched upon incorporation of the ligated ruthenium moiety on proceeding to 11. Third-order nonlinear optical studies of 11 were undertaken at the benchmark wavelengths of 800 nm (fs pulses) and 532 nm (ns pulses), the results from the former suggesting a dominant contribution from two-photon absorption, and results from the latter being consistent with primarily excited-state absorption.