A Combined Experimental and Computational Study of Bond Activation Reactions in Ruthenium N-Heterocyclic Carbene Complexes

过渡金属(TM)络合物在均相催化中的主要应用。这为合成散装化学品(用作溶剂或更复杂产品的原料的相对简单的分子)或精细化学品(如药品)提供了低能量途径。要想成功，TM催化剂必须是稳定、高效和有选择性的。这通常是通过正确选择配体来实现的--配体是一种结合到金属中心以控制反应位置的分子。传统上，膦一直是TM催化的首选配体，尽管在过去的15年里，一类新的配体-N-杂环卡宾(NHCS)已经被开发出来，并且在某些情况下取得了惊人的成功。然而，NHCS的一个缺点是它们本身不是惰性物种，而且还会经历它们自己的反应化学反应。这对催化剂的性能有明显的影响，因为这意味着金属上的反应中心的性质将随着时间的推移而变化，并可能降解。如果要设计以NHC为配体的新一代催化剂，首先了解并控制这种基于NHC的反应活性是至关重要的。这一提议建立在巴斯的惠特尔西小组的初步实验观察的基础上，该小组已经确定了一些基于NHC的不寻常的反应。为了理解这些过程，有必要推断它们发生的机制(S)。TM反应机理通常通过一些单独的步骤发生，这些步骤涉及非常短暂的反应中间体和过渡态。这类物种很难通过实验进行研究，但可以通过计算模型来解决。因此，一种有效的方法是将实验观察(在Whittlesey小组中)与补充计算研究(由Heriot-Watt大学的Macgregor小组进行)相结合，以提供对反应性的额外洞察。在这个提案中，我们将使用实验和计算方法来研究一系列基于NHC的新型反应。我们的目标是使这些进程的机制合理化，以便能够确定发生这些进程的条件。由于这些情况可能与许多TM中心有关，我们希望提供关于TM-NHC物种稳定性的非常一般的信息。最终，我们试图控制基于NHC的反应性，以便可以设计新的催化剂，在那里可以避免这些有害的过程。尽管对催化不利，基于NHC的反应性也可以从更积极的角度来看待。NHC通常通过化学键的断裂来反应，这些化学键通常被认为是“惰性的”，或者至少是高度不反应的。在目前的情况下，观察到了C-N键的断裂反应，通过研究这一反应和相关的(C-H，N-H和Ru-C)键断裂过程，我们的目的也是为了提供关于如何断裂这些键的基本信息。这一洞察力随后可能被用来激活其他(非NHC)系统中的这种化学非反应键。

A major application of transition metal (TM) complexes is in homogeneous catalysis. This provides low energy routes to the synthesis of both bulk chemicals (relatively simple molecules for use as solvents or feedstocks for more complicated products ) or fine chemicals (e.g. pharmaceuticals). To be successful, a TM catalyst must be stable, efficient and selective. This is usually achieved through the correct choice of ligand - a molecule that binds to the metal centre to control the reactive site. Traditionally phosphines have been the ligand of choice in TM catalysis, although in the last 15 years, a new class of ligand, the N-heterocyclic carbenes (NHCs), has been developed and in some cases these have met with spectacular success.One drawback with NHCs, however, is they are not themselves inert species, but will also undergo their own reaction chemistry. This has clear implications for catalyst performance, as it means the nature of the reactive site at the metal will change and possibly degrade with time. It is vital that this NHC-based reactivity is first understood and then controlled if new generations of catalysts featuring NHC as ligands are to be designed. This proposal builds on preliminary experimental observations in the Whittlesey group at Bath that have identified a number of unusual NHC-based reactions. In order to understand these processes it is imperative to deduce the mechanism(s) by which they occur. TM reaction mechanisms often take place through a number of individual steps which involve very short-lived reaction intermediates and transition states. Such species are difficult to study experimentally, but can be tackled using computational modelling. An effective approach is therefore to combine experimental observations (in the Whittlesey group) with complementary computational studies (performed by Macgregor's group at Heriot-Watt University) to provide extra insight into reactivity. In this proposal we will use experimental and computational methods to study a range of novel NHC-based reactions. Our aim is to rationalize the mechanisms of these processes and so to be able to define the conditions under which they occur. As these conditions are likely to pertain with many TM centres, we expect to provide very general information about the stability of TM-NHC species. Ultimately we seek to control NHC-based reactivity so that new catalysts can be designed where these detrimental processes can be avoided.Although detrimental for catalysis, NHC-based reactivity can also be viewed in a more positive light. NHCs often react through the cleavage of chemical bonds that are usually considered 'inert', or at least highly unreactive. In the present case a C-N bond cleavage reaction has been observed and by studying this and related (C-H, N-H and Ru-C) bond cleavage processes we also aim to provide fundamental information on how to break such bonds. This insight may then be used to activate such chemically unreactive bonds in other (non-NHC) systems.