Photochemical [2+2] cycloaddition of the olefinic double bonds in the supramolecular rectangle [Ru₄(η⁶-<i>p</i>-cymene)₄(μ-oxalato)₂(μ-4,4′-bipyridylethylene)₂]⁴⁺

Self-assembly of 4,4′-bipyridylethylene (bpe) bidentate connector with the dinuclear arene ruthenium clip [Ru₂(η⁶-<i>p</i>-cymene)₂(μ-oxalato)Cl₂] in the presence of silver triflate affords the cationic organometallic rectangle [Ru₄(η⁶-<i>p</i>-cymene)₄(μ-oxalato)₂(μ-bpe)₂]⁴⁺ ([<b>1</b>][CF₃SO₃]₄). Upon UV irradiation of a methanol solution of [<b>1</b>]⁴⁺, dimerisation of the olefinic double bonds of the two parallel bpe ligands occurs, thus giving rise to the [2+2] cycloaddition derivative [Ru₄(η⁶-<i>p</i>-cymene)₄(μ-oxalato)₂(μ-tpcb)]⁴⁺ ([<b>2</b>][CF₃SO₃]₄) (tpcb = tetrakis(4-pyridyl)cyclobutane). The [2+2] photodimerisation reaction was followed by ¹H NMR spectroscopy. Moreover, the molecular structure of these two systems was deduced by one-dimensional and two-dimensional NMR experiments (¹H, ¹³C, ROESY, COSY, HSQC). These data suggest the formation of only the <i>rctt</i>-tetrakis(4-pyridyl)cyclobutane isomer.