Fragile Crystals: Anion Control of the Spontaneous Release of Carbon Monoxide from Crystalline Rhodium A-Frame Complexes

Addition of carbon monoxide to violet solutions of [Rh₂(μ-dppm)₂(CN<i>t</i>-Bu)₄]₄(A)₂ (dppm = bis(diphenylphosphino)methane, A = PF₆, SbF₆, BPh₄) (<b>1-3</b>) leads to the formation of yellow crystals of [Rh₂(μ-dppm)₂(CN<i>t</i>-Bu)₄(μ-CO)](PF₆)₂·<i>n</i>CH₂Cl₂ (<b>4</b>), [Rh₂(μ-dppm)₂(CN<i>t</i>-Bu)₄(μ-CO)](SbF₆)₂·<i>n</i>CH₂Cl₂ (<b>5</b>), and [Rh₂(μ-dppm)₂(CN<i>t</i>-Bu)₄(μ-CO)](BPh₄)₂·1.5(CH₃)₂CO (<b>6</b>). Isostructural yellow crystals of (<b>4</b>) or (<b>5</b>) spontaneously crumble and acquire a violet color in a matter of minutes once they are removed from their mother liquor. In contrast, crystals of (<b>6</b>) remain yellow for at least 1 month in air after removal from their mother liquor. The violet color of the transformed crystals of (<b>4</b>) and (<b>5</b>) is due to the formation of the violet dication, [Rh₂(μ-dppm)₂(CN<i>t</i>-Bu)₄]²⁺, as determined by UV/vis and ³¹P{¹H} studies of solutions of transformed crystals. Acetone solutions of (<b>5</b>) lose CO upon heating or purging with dinitrogen, yielding the violet dication [Rh₂(μ-dppm)₂(CN<i>t</i>-Bu)₄]²⁺. The structures of violet (<b>2</b>), yellow (<b>4</b>), yellow (<b>5</b>), and yellow (<b>6</b>) have been determined by single-crystal X-ray diffraction. Yellow crystals of (<b>4</b>) and (<b>5</b>) contain channels that facilitate the loss of carbon monoxide. No such channels are present in (<b>6</b>).