Mixed-Metal Cluster Chemistry. 30. Syntheses and Optical Limiting Properties of Cluster-Containing Oligo- and Polyurethanes

Abstract

Molybdenum−iridium clusters Mo2Ir2{μ4-η2-C2[(CH2)6OH]2}(μ-CO)4(CO)4(η-C5H4R)2 [R = H (5), Me (6), (CH2)9O(THP) (7); THP = tetrahydropyranyl] are formed from reaction of Mo2Ir2(μ-CO)3(CO)7(η-C5H4R)2 with the alkyne HO(CH2)6C2(CH2)6OH. Reaction of 6 and 7 with the diisocyanate OCN(CH2)4NCO affords oligourethanes [−O(CH2)6(μ4-η2-C2){Mo2Ir2(μ-CO)4(CO)4(η-C5H4R)2}(CH2)6OC(O)NH(CH2)4NHC(O)−]n [R = Me (12), (CH2)9O(THP) (13)], with transition metal clusters in the backbone, the extent of polymerization being ascertained by size exclusion chromatography and 1H NMR spectroscopy. Characterization of the cluster-containing oligo- and polyurethanes was aided by 1H NMR and IR spectral comparison with model cluster diurethanes Mo2Ir2{μ4-η2-C2[(CH2)6OC(O)NH(CH2)3Me]2}(μ-CO)4(CO)4(η-C5H4R)2 [R = H (8), Me (9), (CH2)9O(THP) (11)], prepared from reaction between the cluster diols 5, 6, and 7 and the isocyanate Me(CH2)3NCO. The diol cluster 5 and cluster diurethane 8 have been characterized by single-crystal X-ray diffraction studies. Removal of the THP groups in 13 gives [−O(CH2)6(μ4-η2-C2){Mo2Ir2(μ-CO)4(CO)4(η-C5H4(CH2)9OH)2}(CH2)6OC(O)NH(CH2)4NHC(O)−]n (14), which has been cross-linked on reaction with OCN(CH2)4NCO. The cluster content of the polyurethanes can be diluted; polymerization of 1:19 6:1,6-hexanediol and OCN(CH2)4NCO affords copolymer [−O(CH2)6(μ4-η2-C2){Mo2Ir2(μ-CO)4(CO)4(η-C5H4Me)2}(CH2)6OC(O)NH(CH2)4NHC(O){O(CH2)6OC(O)NH(CH2)4NHC(O)}m−]n (16). The optical limiting properties of these dimolybdenum-diiridium clusters and cluster-containing polymers, and related molybdenum-triiridium clusters and tungsten-containing homologues, have been assessed by open-aperture Z-scan studies at 527 nm employing ns pulses. Optical limiting merit increases upon increasing group 6 metal content, and replacing the 4d metal molybdenum with the 5d metal tungsten. Some insight into the mechanism by which these clusters function as optical limiters has been gleaned from complementary pump−probe studies, with their behavior consistent with a fast nonlinear absorption process followed by reverse saturable absorption involving metastable excited states of greater than nanosecond lifetime.