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(CNt-Bu)₄]₄(A)₂ (dppm = bis(diphenylphosphino)methane, A = PF₆, SbF₆, BPh₄) (1-3) leads to the formation of yellow crystals of [Rh₂(μ-dppm)₂(CNt-Bu)₄(μ-CO)](PF₆)₂·nCH₂Cl₂ (4), [Rh₂(μ-dppm)₂(CNt-Bu)₄(μ-CO)](SbF₆)₂·nCH₂Cl₂ (5), and [Rh₂(μ-dppm)₂(CNt-Bu)₄(μ-CO)](BPh₄)₂·1.5(CH₃)₂CO (6). Isostructural yellow crystals of (4) or (5) spontaneously crumble and acquire a violet color in a matter of minutes once they are removed from their mother liquor. In contrast, crystals of (6) remain yellow for at least 1 month in air after removal from their mother liquor. The violet color of the transformed crystals of (4) and (5) is due to the formation of the violet dication, [Rh₂(μ-dppm)₂(CNt-Bu)₄]²⁺, as determined by UV/vis and ³¹P{¹H} studies of solutions of transformed crystals. Acetone solutions of (5) lose CO upon heating or purging with dinitrogen, yielding the violet dication [Rh₂(μ-dppm)₂(CNt-Bu)₄]²⁺. The structures of violet (2), yellow (4), yellow (5), and yellow (6) have been determined by single-crystal X-ray diffraction. Yellow crystals of (4) and (5) contain channels that facilitate the loss of carbon monoxide. No such channels are present in (6).