Department of Chemistry, University of California, Davis, California 95616, USA.
Inorg Chem. 2012 Aug 20;51(16):8997-9004. doi: 10.1021/ic301128m. Epub 2012 Jul 27.
Redox active ligands are shown to facilitate a variety of group transfer reactions at redox inert aluminum(III). Disulfides can be used as a two-electron group transfer reagent, and we show that (IP(-))(2)AlSR can be formed by reaction of [(THF)(6)Na][(IP(2-))(2)Al] (1c) with disulfides RSSR (where X = C(S)NMe(2), 4; SMe, 5). In a more general redox route to substitution of aluminum bis(iminopyridine) complexes, we report zinc(II) salts as a group transfer reagent. Reaction of ((R)IP(2-))(2)Al (R = H, 1c; Me, 1d) with ZnX(2) affords ((R)IP(-))(2)AlX (where IP = iminopyridine, R = H, and X = Cl, 2; CCPh, 6; N(3), 7; SPh, 8; or R = Me and X = NHPh, 9). Single crystal X-ray diffraction analysis of the complexes reveal that each of the five coordinate complexes reported here has a trigonal bipyramidal geometry with τ = 0.668 - 0.858. We observed a correlation between the greatest deviations from ideal trigonal bipyramidal symmetry (lowest τ values), the bond lengths consistent with smallest degree of ligand reduction, and the least polarizable X ligand in (IP(-))(2)AlX. Complex 4 is six-coordinate and is best described as distorted octahedral. Variable temperature magnetic susceptibility measurements indicate that each of the complexes 3-9 has a biradical electronic structure similar to previously reported 2. Magnetic exchange coupling constants in the range J = -94 to -212 cm(-1) were fit to the data for 2-9 to describe the energy of antiferromagnetic interaction between ligand radicals assuming a spin Hamiltonian of the form Ĥ = -2JŜ(L(1))·Ŝ(L(2)). The strongest coupling occurs when the angle between the ligand planes is smallest, presumably to afford good overlap with the Al-X σ* orbital. Electrochemical properties of the complexes were probed using cyclic voltammetry and each of 3-9 displayed a reversible two-electron reduction and two quasi-reversible one-electron oxidation processes. The energy of the ligand based redox processes for 2-9 differ by about 150 mV over all complexes and show a correlation with the degree of IP(-) reduction observed crystallographically; more reduced IP(-) ligands require higher potentials for further reduction. Comproportionation constants that describe the equilibrium for the reaction (IP(-))(2)AlX + (IP)(2)AlX ↔ (IP(-))(IP)AlX fall in the range of K(c) = 10(5.7) to 10(7.9) for 3-9.
氧化还原活性配体被证明可以促进各种在氧化还原惰性铝（III）上的基团转移反应。二硫键可用作双电子基团转移试剂，我们表明，（IP(-))(2)AlSR 可以通过反应[(THF)(6)Na][(IP(2-))(2)Al]（1c）与二硫键 RSSR（其中 X = C(S)NMe(2), 4；SMe, 5）形成。在一种更通用的取代铝双（异吡咯）配合物的氧化还原途径中，我们报告锌（II）盐作为基团转移试剂。((R)IP(2-))(2)Al（R = H, 1c；Me, 1d）与 ZnX(2)反应得到((R)IP(-))(2)AlX（其中 IP = 异吡咯啶，R = H，X = Cl, 2；CCPh, 6；N(3), 7；SPh, 8；或 R = Me，X = NHPh, 9）。所报道的五个配位配合物的单晶 X 射线衍射分析表明，这里的每个配合物都具有三角双锥几何形状，τ = 0.668-0.858。我们观察到从理想三角双锥对称性的最大偏差（最低 τ 值）与键长之间存在相关性，与配体还原度最小一致，并且 X 配体在（IP(-))(2)AlX 中最不可极化。配合物 4 是六配位的，最好描述为扭曲的八面体。变温磁化率测量表明，配合物 3-9 中的每个化合物都具有与先前报道的 2 类似的双自由基电子结构。在 2-9 中拟合磁交换耦合常数在 -94 到 -212 cm(-1) 的范围内以描述配体自由基之间的反铁磁相互作用的能量，假设自旋哈密顿形式为 Ĥ = -2JŜ(L(1))·Ŝ(L(2))。当配体平面之间的角度最小时，发生最强的耦合，这可能是为了与 Al-X σ*轨道很好地重叠。使用循环伏安法研究了配合物的电化学性质，3-9 中的每一个都显示出可逆的两电子还原和两个准可逆的单电子氧化过程。2-9 中配体基的氧化还原过程的能量相差约 150 mV，在所有配合物中均表现出相关性，并且与晶体学上观察到的 IP(-)还原程度相关；更还原的 IP(-)配体需要更高的电位才能进一步还原。描述反应（IP(-))(2)AlX + (IP)(2)AlX ↔ (IP(-))(IP)AlX 平衡的比例常数在 3-9 的范围为 K(c) = 10(5.7) 到 10(7.9)。
