Synthesis and Characterization of Ruthenium Bis(β-diketonato) Pyridine-Imidazole Complexes for Hydrogen Atom Transfer

Ruthenium bis(β-diketonato) complexes have been prepared at both the RuII and RuIII oxidation levels and with
protonated and deprotonated pyridine−imidazole ligands. RuII(acac)2(py-imH) (1), [RuIII(acac)2(py-imH)]OTf (2), RuIII(acac)2(py-im) (3), RuII(hfac)2(py-imH) (4), and [DBU−H][RuII(hfac)2(py-im)] (5) have been fully characterized, including
X-ray crystal structures (acac = 2,4-pentanedionato, hfac = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionato, py-imH =
2-(2‘-pyridyl)imidazole, DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene). For the acac-imidazole complexes 1 and 2,
cyclic voltammetry in MeCN shows the RuIII/II reduction potential (E1/2) to be −0.64 V versus Cp2Fe+/0. E1/2 for the
deprotonated imidazolate complex 3 (−1.00 V) is 0.36 V more negative. The RuII bis-hfac analogues 4 and 5 show
the same ΔE1/2 = 0.36 V but are 0.93 V harder to oxidize than the acac derivatives (0.29 and −0.07 V). The
difference in acidity between the acac and hfac derivatives is much smaller, with pKa values of 22.1 and 19.3 in
MeCN for 1 and 4, respectively. From the E1/2 and pKa values, the bond dissociation free energies (BDFEs) of the
N−H bonds in 1 and 4 are calculated to be 62.0 and 79.6 kcal mol-1 in MeCN − a remarkable difference of
17.6 kcal mol-1 for such structurally similar compounds. Consistent with these values, there is a facile net hydrogen
atom transfer from 1 to TEMPO• (2,2,6,6-tetramethylpiperidine-1-oxyl radical) to give 3 and TEMPO-H. The ΔG°
for this reaction is −4.5 kcal mol-1. 4 is not oxidized by TEMPO• (ΔG° = +13.1 kcal mol-1), but in the reverse
direction TEMPO-H readily reduces in situ generated RuIII(hfac)2(py-im) (6). A RuII-imidazoline analogue of 1, RuII(acac)2(py-imnH) (7), reacts with 3 equiv of TEMPO• to give the imidazolate 3 and TEMPO-H, with dehydrogenation
of the imidazoline ring.

双(β-二酮合)钌配合物（Ruthenium bis(β-diketonato) complexes）已分别在Ru(II)与Ru(III)氧化态下，以质子化及去质子化的吡啶-咪唑配体制得。其中，Ru(II)(acac)₂(py-imH)（1，其中acac为2,4-戊二酮合基，py-imH为2-(2'-吡啶基)咪唑）、[Ru(III)(acac)₂(py-imH)]OTf（2，OTf为三氟甲磺酸盐）、Ru(III)(acac)₂(py-im)（3）、Ru(II)(hfac)₂(py-imH)（4，hfac为1,1,1,5,5,5-六氟-2,4-戊二酮合基）以及[DBU-H][Ru(II)(hfac)₂(py-im)]（5，DBU为1,8-二氮杂双环[5.4.0]十一碳-7-烯）已得到完整表征，涵盖X射线晶体结构测定。
针对acac-咪唑配合物1和2，在乙腈(MeCN)中开展的循环伏安测试显示，其Ru(III)/Ru(II)的半波还原电势(E1/2)相对于二茂铁阳离子/中性偶(Cp₂Fe⁺/0)为-0.64 V。去质子化的咪唑基配合物3的E1/2为-1.00 V，较前者负移0.36 V。双(hfac)合钌(II)类似物4和5展现出相同的ΔE1/2=0.36 V，但相较于acac类衍生物，其氧化难度高出0.93 V（半波电势分别为0.29 V和-0.07 V）。
acac与hfac衍生物的酸度差异相对较小，在乙腈溶剂中，配合物1和4的pKa值分别为22.1和19.3。基于E1/2与pKa数值，可计算得到1和4中N-H键的键解离自由能(BDFEs)在乙腈中分别为62.0和79.6 kcal·mol⁻¹——对于结构极为相似的这类化合物而言，17.6 kcal·mol⁻¹的差值十分显著。
与上述计算结果相符，配合物1可与TEMPO•(2,2,6,6-四甲基哌啶-1-氧基自由基)发生高效的净氢原子转移反应，生成3与TEMPO-H。该反应的标准吉布斯自由能变(ΔG°)为-4.5 kcal·mol⁻¹。配合物4无法被TEMPO•氧化(ΔG°=+13.1 kcal·mol⁻¹)，但在逆反应方向上，TEMPO-H可轻松还原原位生成的Ru(III)(hfac)₂(py-im)（6）。
配合物1的Ru(II)-咪唑啉类似物Ru(II)(acac)₂(py-imnH)（7）可与3当量的TEMPO•反应，生成咪唑基配合物3与TEMPO-H，同时伴随咪唑啉环的脱氢过程。