In search of a fundamental understanding of C-H & C-C agostic bonds

电子不饱和过渡金属中心和饱和配体之间的相互作用“agostic”键的发现从根本上改变了有机金属化学家对结构，机理和催化的思考方式。Agostic“相互作用”的结构后果是清楚的：配体和配位球扭曲，使C-H键可以接近金属中心。然而，这种相互作用的根本原因并不那么明显，至少有四种不同的电子机制被提出来解释这种扭曲。其中最早也是最被广泛接受的是C-H键向金属提供两个电子，但这个简单的模型未能合理化Agostic键的许多更微妙的方面。因此，一些作者提出，键合电子的离域是M-C，而不是C-H键合电子的关键，而其他人则认为电荷从金属转移到C-H基团上的反键轨道。显然，“agostic相互作用”是一个非常普遍的概念，它涵盖了广泛的电子机制，并且每个电子机制的相对重要性可能因情况而异。在与图卢兹的Michel Etienne实验组的长期合作过程中，我们表征了一种环丙基络合物，在我们看来，这是无约束系统中C-C agostic相互作用的第一个例子。在定性方面，没有理由饱和的C-C键不应该参与与C-H键相同类型的相互作用。然而，在绝大多数情况下，由于空间和电子原因，C-H激动剂替代物是优选的，并且先前报道的C-C激动剂的唯一实例发生在没有可行的CH激动剂替代物的情况下。相比之下，潜在的agostic C-H键很容易在环丙基配体，这表明一个真正的电子偏好的C-C agostic结构在这种情况下，这种全新的键合模式的发现立即提出了问题的精确性质的C-C agostic相互作用。它的电子起源是否与更常见的C-H agostics相同，或者不同稳定机制之间的平衡是否根本不同？在这个提议中，我们的目标是使用最先进的计算技术来探索这些问题，这些技术使我们能够将电子的详细分布放在显微镜下。通过检查的电子结构，也，在可用的情况下，仔细比较与实验，我们的目标是统一的术语“agostic”的不同解释，并提供这些相互作用的基本性质的亲密的理解。

The discovery of the 'agostic' bond, an interaction between an electronically unsaturated transition metal centre and a saturated ligand, has radically altered the way in which organometallic chemists think about structure, mechanism and catalysis. The structural consequences of the agostic 'interaction' are clear: the ligand and the coordination sphere distort so that a C-H bond can approach the metal centre. The underlying origins of the interaction are, however, rather less obvious, and at least four distinct electronic mechanisms have been proposed to explain the distortions. The earliest, and most widely accepted, of these is the donation of two electrons from the C-H bond to the metal, but this simple model fails to rationalise many of the more subtle aspects of agostic bonding. As a result, some authors have proposed that delocalisation of the M-C, rather than C-H, bonding electrons is the key, while others have invoked charge transfer from metal to the antibonding orbital on the C-H group. Clearly, then, the term 'agostic interaction' is a very general one that encompasses a broad range of electronic mechanisms, and it is possible that the relative importance of each of these varies from case to case.During the course of a long-standing collaboration with the experimental group of Michel Etienne in Toulouse, we have characterised a cyclopropyl complex that represents, in our view, the first example of a C-C agostic interaction in an unconstrained system. In qualitative terms, there is no reason why a saturated C-C bond should not participate in the same types of interactions as a C-H bond. The C-H agostic alternative is, however, preferred in the vast majority of cases for steric and electronic reasons, and the only previously reported examples of C-C agostics occur where there is no viable CH agostic alternative. In contrast, potentially agostic C-H bonds are readily accessible in the cyclopropyl ligand, suggesting a genuine electronic preference for a C-C agostic structure in this case.The discovery of this fundamentally new bonding mode immediately raises questions about the precise nature of the C-C agostic interaction. Are its electronic origins the same as the more common C-H agostics, or is the balance between the different stabilising mechanisms fundamentally different? In this proposal, we aim to explore these questions using state-of-the art computational techniques that allow us to place the detailed distribution of electrons under the microscope. Through an examination of the electronic structure, and also, where available, careful comparison with experiment, we aim to unify the different explanations of the term 'agostic', and offer an intimate understanding of the fundamental nature of these interactions.