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 [Rh 2 (μ-dppm) 2 (CN t -Bu) 4 ] 4 (A) 2 (dppm = bis(diphenylphosphino)methane, A = PF 6, SbF 6, BPh 4 ) ( 1–3 ) leads to the formation of yellow crystals of [Rh 2 (μ-dppm) 2 (CN t -Bu) 4 (μ-CO)](PF 6 ) 2 · n CH 2 Cl 2 ( 4 ), [Rh 2 (μ-dppm) 2 (CN t -Bu) 4 (μ-CO)](SbF 6 ) 2 · n CH 2 Cl 2 ( 5 ), and [Rh 2 (μ-dppm) 2 (CN t -Bu) 4 (μ-CO)](BPh 4 ) 2 ·1.5(CH 3 ) 2 CO ( 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, [Rh 2 (μ-dppm) 2 (CN t -Bu) 4 ] 2+, as determined by UV/vis and 31 P{ 1 H} studies of solutions of transformed crystals. Acetone solutions of ( 5 ) lose CO upon heating or purging with dinitrogen, yielding the violet dication [Rh 2 (μ-dppm) 2 (CN t -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 ).