[2+2] Photodimerization in the Solid State Aided by Molecular Templates of Rectangular Macrocycles Bearing Oxamidato Ligands

A series of binuclear half-sandwich iridium and rhodium complexes bearing oxamidato ligands [Cp* 2 M 2 (μ- N, N ′-bis(aryl)oxamidato)Cl 2 ] ( 1a, M = Ir, aryl = Ph; 1b, M = Ir, aryl = C 6 H 4 - p -Me; 2a, M = Rh, aryl = Ph; 2b, M = Rh, aryl = C 6 H 4 - p -Me) functioned as potential ‘‘organometallic clip’’ linear templates were synthesized by the reactions of the lithium salts of oxamide with [Cp*MCl 2 ] 2 (M = Ir or Rh), respectively. Treatment of the binuclear complexes ( 1a − 2b ) with trans -1,2-bis(4-pyridyl)ethylene (4,4′-bpe) in the presence of AgOTf (OTf = CF 3 SO 3 ) gave the corresponding tetranuclear complexes of general formula [Cp* 4 M 4 (μ- N, N ′-bis(aryl)oxamidato) 2 (μ-4,4′-bpe) 2 ](OTf) 4 ( 3a, M = Ir, aryl = Ph; 3b, M = Ir, aryl = C 6 H 4 - p -Me; 4a, M = Rh, aryl = Ph; 4b, M = Rh, aryl = C 6 H 4 - p -Me) in high yields. Confirmed by the single-crystal X-ray analysis, the molecular structures of tetranuclear complexes 3a and 4a showed that two binuclear fragments as building blocks were connected by μ-4,4′-bpe to construct a rectangular cavity with the dimensions 5.58 × 13.59 Å ( 3a ) and 5.55 × 13.65 Å ( 4a ). The two μ-4,4′-bpe ligands in 3a and 4a are close to each other within 4.2 Å to allow the [2+2] photoreaction in the solid state, which was proved by 1 H NMR of the resulting products ( 5a − 6b′ ) and single-crystal X-ray analysis.