Photocatalytic H2-Evolution by Homogeneous Molybdenum Sulfide Clusters Supported by Dithiocarbamate Ligands.
Photocatalytic H2-Evolution by Homogeneous Molybdenum Sulfide Clusters Supported by Dithiocarbamate Ligands.
复制标题
二硫代氨基甲酸酯配体支持的均质硫化钼簇光催化析氢。
作者：
P. Fontenot;B. Shan;Bo Wang;Spenser R Simpson;Gayathri Ragunathan;Angelique F Greene;Antony Obanda;Leigh Anna Hunt;N. Hammer;C. E. Webster;J. Mague;R. Schmehl;J. P. Donahue
[Mo 3（μ3-S）（μ2-S2）3（S2 CNR 2）3]I（[2a]I，R = Me; [2b]I，R = Et; [2c]I，R = iBu; [2d]I，R = CH 2C 6 H5）在混合水极性有机介质中以[Ru（bipy）3]2+作为光敏剂和Et 3 N作为电子供体在460 nm处辐照导致H 2释放。最大活性（300次周转，3小时），发现与R = iBu在1：9 H2O：MeCN;减少的活动是由于恶化的[Ru（bipy）3]2+。通过质谱法监测光解混合物表明，[Mo 3（μ3-S）（μ2-S2）3（S2 CNR 2）3]+通过在几分钟的时间尺度上挤出硫而转化为[Mo 3（μ3-S）（μ2-S）3（S2 CNR 2）3]+，而没有中间体[Mo 3S 6（S2 CNR 2）3]+或[Mo 3S 5（S2 CNR 2）3]+物质的积累。有意制备[Mo 3S 4（S2 CNEt 2）3]+（[3]+）并用Et 2NCS 21-处理产生[Mo 3S 4（S2 CNEt 2）4]（4），其中第四个二硫代氨基甲酸酯配体桥接Mo 3三角形的一个边缘。4的光解导致H2释放，但在[Mo 3S 7（S2 CNEt 2）3]+观察到的水平的约25%处。对光解的早期监测表明，[Mo 3S 4（S2 CNEt 2）4]立即以恒定速率放出H2，而[Mo 3S 7（S2 CNEt 2）3]+在更快的H2放出速率之前显示出独特的孵育。这一观察结果意味着在这两种情况下不同特性的催化剂的作用。[Mo 3S 7]的光解溶液（S2 CNiBu 2）3]+在24小时内保持不受干扰存款不对称的Mo 6簇[（iBu2NCS2）3（μ2-S2）2（μ3-S）Mo3]（μ3-S）（μ3-η2，η1-S'，η1-S″-S2）[Mo 3（μ2-S）3（μ3-S）]（S2CNiBu2）2（μ2-S2CNiBu2）]，这表明具有这种六金属组成和核心拓扑结构的物种可能是以[Mo 3S 7（S2 CNR 2）3]+开始的光解中的放氢催化剂.当用作溶剂时，N，N-二甲基甲酰胺（DMF）抑制H2-放出，但对于[Mo 3S 4（S2 CNEt 2）4]比对于[Mo 3S 7（S2 CNEt 2）3]+抑制程度更大。[Mo 3S 4（S2 CNEt 2）4]从DMF中重结晶得到[Mo 3S 4（S2 CNEt 2）4（η1，κO-DMF）]（5），这意味着DMF的抑制作用来自对Mo配位位点的竞争，该位点是H2释放所必需的。在添加2 H+和2 e-后对[Mo 3S 4（S2 CNMe 2）3]+的计算评估表明Mo（H）-μ2（SH）中间体是H2消除的最低能量物质。类似的途径可能通过解离[Mo 3S 4]4+片段中μ2-S2 CNR 2配体（一种已知的半不稳定配体类型）的一端而对Mo 6簇可用。
Irradiation at 460 nm of [Mo3(μ3-S)(μ2-S2)3(S2CNR2)3]I ([2a]I, R = Me; [2b]I, R = Et; [2c]I, R = iBu; [2d]I, R = CH2C6H5) in a mixed aqueous-polar organic medium with [Ru(bipy)3]2+ as photosensitizer and Et3N as electron donor leads to H2 evolution. Maximum activity (300 turnovers, 3 h) is found with R = iBu in 1:9 H2O:MeCN; diminished activity is attributed to deterioration of [Ru(bipy)3]2+. Monitoring of the photolysis mixture by mass spectrometry suggests transformation of [Mo3(μ3-S)(μ2-S2)3(S2CNR2)3]+ to [Mo3(μ3-S)(μ2-S)3(S2CNR2)3]+ via extrusion of sulfur on a time scale of minutes without accumulation of the intermediate [Mo3S6(S2CNR2)3]+ or [Mo3S5(S2CNR2)3]+ species. Deliberate preparation of [Mo3S4(S2CNEt2)3]+ ([3]+) and treatment with Et2NCS21- yields [Mo3S4(S2CNEt2)4] (4), where the fourth dithiocarbamate ligand bridges one edge of the Mo3 triangle. Photolysis of 4 leads to H2 evolution but at ∼25% the level observed for [Mo3S7(S2CNEt2)3]+. Early time monitoring of the photolyses shows that [Mo3S4(S2CNEt2)4] evolves H2 immediately and at constant rate, while [Mo3S7(S2CNEt2)3]+ shows a distinctive incubation prior to a more rapid H2 evolution rate. This observation implies the operation of catalysts of different identity in the two cases. Photolysis solutions of [Mo3S7(S2CNiBu2)3]+ left undisturbed over 24 h deposit the asymmetric Mo6 cluster [(iBu2NCS2)3(μ2-S2)2(μ3-S)Mo3](μ3-S)(μ3-η2,η1-S',η1-S″-S2)[Mo3(μ2-S)3(μ3-S)(S2CNiBu2)2(μ2-S2CNiBu2)] in crystalline form, suggesting that species with this hexametallic composition and core topology are the probable H2-evolving catalysts in photolyses beginning with [Mo3S7(S2CNR2)3]+. When used as solvent, N,N-dimethylformamide (DMF) suppresses H2-evolution but to a greater degree for [Mo3S4(S2CNEt2)4] than for [Mo3S7(S2CNEt2)3]+. Recrystallization of [Mo3S4(S2CNEt2)4] from DMF affords [Mo3S4(S2CNEt2)4(η1,κO-DMF)] (5), implying that inhibition by DMF arises from competition for a Mo coordination site that is requisite for H2 evolution. Computational assessment of [Mo3S4(S2CNMe2)3]+ following addition of 2H+ and 2e- suggests a Mo(H)-μ2(SH) intermediate as the lowest energy species for H2 elimination. An analogous pathway may be available to the Mo6 cluster via dissociation of one end of the μ2-S2CNR2 ligand, a known hemilabile ligand type, in the [Mo3S4]4+ fragment.