Substituent Effects on the Electrochemical, Spectroscopic, and Structural Properties of Fischer Mono- and Biscarbene Complexes of Chromium(0)

A series
of ten ferrocenyl, furyl, and thienyl mono- and biscarbene
chromium(0) complexes were synthesized and characterized spectroscopically
and electrochemically. The single crystal structure of the biscarbene
complex [(CO)5CrC­(OEt)-Fu′-(OEt)­CCr­(CO)5] (4a) was determined: C20H12Cr2O13; triclinic; P1̅; a = 6.2838(5), b = 12.6526(9), c = 29.1888(19) Å, α = 89.575(2), β = 88.030(2),
γ = 87.423(2)°; Z = 4. Results from an
electrochemical study in CH2Cl2 were mutually
consistent with a computational study in showing that the carbene
double bond of 1 – 6 is reduced to
an anion radical, –Cr–C• at formal
reduction potentials < −1.7 V vs FcH/FcH+. The
Cr centers are oxidized in two successive one electron transfer steps
to Cr­(II) via the Cr­(I) intermediate. Only Cr­(I) oxidation is electrochemically
irreversible. Dicationic Cr­(II) species formed upon two consecutive
one-electron oxidation processes are characterized by a peculiar bonding
situation as they are stabilized by genuine CH···Cr
agostic interactions. With respect to aryl substituents, carbene redox
processes occurred at the lowest potentials for ferrocene derivatives
followed by furan complexes. Redox process in the thiophene derivatives
occurred at the highest potentials. This result is mutually consistent
with a 13C NMR study that showed the CrC functionality
of furyl complexes were more shielded than thienyl complexes. The
NHBu carbene substituent resulted in carbene complexes showing redox
processes at substantially lower redox potentials than carbenes having
OEt substituents.

本研究合成了10种二茂铁基（ferrocenyl）、呋喃基（furyl）及噻吩基（thienyl）单卡宾与双卡宾铬(0)配合物，并通过光谱及电化学手段对其进行了表征。本研究测定了双卡宾配合物[(CO)5Cr=C(OEt)-Fu′-(OEt)C=Cr(CO)5]（4a）的单晶结构：分子式为C20H12Cr2O13，晶体属三斜晶系，空间群为P$overline{1}$，晶胞参数a=6.2838(5) Å、b=12.6526(9) Å、c=29.1888(19) Å，α=89.575(2)°、β=88.030(2)°、γ=87.423(2)°，晶胞内分子数Z=4。在二氯甲烷（CH2Cl2）溶剂中开展的电化学研究结果与计算化学研究结果一致，均表明编号为1~6的配合物，其卡宾双键会在相对于二茂铁/二茂铁正离子（FcH/FcH+）的正式还原电位低于-1.7 V时，被还原为阴离子自由基-Cr-C•。铬中心会经历两步连续的单电子转移氧化过程，经由Cr(I)中间体最终被氧化为Cr(II)；其中仅Cr(I)的氧化过程表现出电化学不可逆性。经两步连续单电子氧化过程生成的二价阳离子Cr(II)物种，其成键环境较为特殊，可通过真实存在的CH···Cr agostic相互作用（agostic interactions）得到稳定。对于芳基取代基而言，二茂铁衍生物的卡宾氧化还原过程所需电位最低，其次为呋喃类配合物，而噻吩类衍生物的氧化还原过程所需电位最高。该结果与碳-13核磁共振（13C NMR）研究结果一致，后者表明呋喃基配合物中的Cr=C官能团相较于噻吩基配合物，其碳信号受到更强的屏蔽。带有NHBu卡宾取代基的配合物，其氧化还原过程所需电位远低于带有乙氧基（OEt）取代基的卡宾配合物。