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

“agostic”键的发现，即电子不饱和过渡金属中心与饱和配体之间的相互作用，从根本上改变了有机金属化学家对结构、机制和催化的思考方式。agostic“相互作用”的结构后果是明确的：配体和配位球扭曲，使碳氢键可以接近金属中心。然而，这种相互作用的潜在起源却不那么明显，至少有四种不同的电子机制被提出来解释这种扭曲。最早的，也是最被广泛接受的，是碳氢键给金属两个电子，但是这个简单的模型不能解释原子成键的许多更微妙的方面。因此，一些作者提出，键电子M-C而非碳氢键电子的离域是关键，而另一些人则认为电荷从金属转移到碳氢基团的反键轨道上。显然，“有机相互作用”一词是一个非常普遍的词，它包含了广泛的电子机制，并且每种机制的相对重要性可能因情况而异。在与图卢兹Michel Etienne研究组的长期合作过程中，我们已经表征了一个环丙基配合物，在我们看来，它代表了无约束系统中C-C有机相互作用的第一个例子。从定性的角度来看，饱和的碳碳键没有理由不参与与碳氢键相同类型的相互作用。然而，由于空间和电子原因，在绝大多数情况下，C-H超声替代方案是首选的，并且之前报道的C-C超声的唯一例子发生在没有可行的CH超声替代方案的情况下。相反，潜在的有机碳-氢键在环丙基配体中很容易接近，这表明在这种情况下，碳-碳有机结构具有真正的电子偏好。这种全新键合模式的发现立即引发了关于碳-碳原子相互作用的确切性质的问题。它的电子起源与更常见的碳氢化合物的起源是一样的，还是不同稳定机制之间的平衡根本不同？在本提案中，我们的目标是利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在显微镜下放置电子的详细分布。通过对电子结构的检查，以及在可能的情况下与实验的仔细比较，我们的目标是统一对“agostic”一词的不同解释，并提供对这些相互作用的基本性质的深入理解。

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.