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Fragile Crystals: Anion Control of the Spontaneous Release of Carbon Monoxide from Crystalline Rhodium A-Frame
Steven R Gralinski1, Sarah Costa1, James C Fettinger1
1Department of Chemistry, University of California, Davis, 1 Shields Avenue, Davis, California 95616, United States.
Abstract:
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).
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