[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*2M2(μ-N,N′-bis(aryl)oxamidato)Cl2] (1a, M = Ir, aryl = Ph; 1b, M = Ir, aryl = C6H4-p-Me; 2a, M = Rh, aryl = Ph; 2b, M = Rh, aryl = C6H4-p-Me) functioned as potential ‘‘organometallic clip’’ linear templates were synthesized by the reactions of the lithium salts of oxamide with [Cp*MCl2]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 = CF3SO3) gave the corresponding tetranuclear complexes of general formula [Cp*4M4(μ-N,N′-bis(aryl)oxamidato)2(μ-4,4′-bpe)2](OTf)4 (3a, M = Ir, aryl = Ph; 3b, M = Ir, aryl = C6H4-p-Me; 4a, M = Rh, aryl = Ph; 4b, M = Rh, aryl = C6H4-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 1H NMR of the resulting products (5a−6b′) and single-crystal X-ray analysis.