Energetics of Variable Hapticity of Carbocyclic Rings in Cyclopentadienylmetal Carbonyl Systems of the Second Row Transition Metals C5H5M(CO)nCmHm (M = Ru, Tc, Mo, Nb) Including Mechanistic Studies of Carbonyl Dissociation
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Decarbonylation of the experimentally known CpRu(CO)2(η1-C5H5), CpMo(CO)2(η3-C7H7), and CpNb(CO)2(η4-C8H8) (Cp = η5-C5H5), each with uncomplexed 1,3-butadiene units in the CnHn ring, as well as the related CpTc(CO)2(η2-C6H6), to give the corresponding carbonyl-free derivatives CpM(ηn-CnHn) derivatives has been studied by density functional theory. For ruthenium, technetium, and molybdenum the coordinated CnHn ring of the intermediate monocarbonyl CpM(CO)(ηn–2-CnHn) contains an uncomplexed CC double bond and each decarbonylation step proceeds with a significant energy barrier represented by a higher energy transition state. However, decarbonylation of CpNb(CO)2(η4-C8H8) to the monocarbonyl proceeds without an energy barrier, preserving the tetrahapto coordination of the C8H8 ring to give CpNb(CO)(η4-C8H8) in which the niobium atom has only a 16-electron configuration. All of the monocarbonyl derivatives CpM(CO)(CnHn) are predicted to be strongly energetically disfavored with respect to disproportionation to give CpM(CO)2(CnHn) + CpM(CnHn). This allows us to understand the failure to date to synthesize any of the monocarbonyl derivatives.
本研究采用密度泛函理论（density functional theory），对实验中已报道的CpRu(CO)₂(η¹-C₅H₅)、CpMo(CO)₂(η³-C₇H₇)及CpNb(CO)₂(η⁴-C₈H₈）（其中Cp为η⁵-C₅H₅，即环戊二烯基），以及相关的CpTc(CO)₂(η²-C₆H₆）的脱羰基反应展开了系统探究。上述配合物的CₙHₙ环中均含有未配位的1,3-丁二烯单元，反应最终生成对应的无羰基衍生物CpM(ηⁿ-CₙHₙ）。 对于钌、锝和钼的配合物，其单羰基中间体CpM(CO)(ηⁿ⁻²-CₙHₙ）的配位CₙHₙ环中存在一个未配位的C=C双键，且每一步脱羰基反应均需跨越具有显著能垒的过渡态。 然而，CpNb(CO)₂(η⁴-C₈H₈）脱羰基生成单羰基中间体的过程无明显能垒，反应过程中保留了C₈H₈环的四齿配位模式，最终得到CpNb(CO)(η⁴-C₈H₈），此时铌原子仅呈现16电子构型。 所有单羰基衍生物CpM(CO)(CₙHₙ）在能量上均极不利于发生歧化反应，生成CpM(CO)₂(CₙHₙ）与CpM(CₙHₙ）的混合物，这也合理解释了截至目前尚未成功合成任何单羰基衍生物的原因。
