The term agostic, derived from the Ancient Greek word for "to hold close to oneself", was coined by Maurice Brookhart and Malcolm Green, on the suggestion of the classicistJasper Griffin, to describe this and many other interactions between a transition metal and a C−H bond. Often such agostic interactions involve alkyl or aryl groups that are held close to the metal center through an additional σ-bond.[2][3]
Short interactions between hydrocarbon substituents and coordinatively unsaturated metal complexes have been noted since the 1960s. For example, in tris(triphenylphosphine) ruthenium dichloride, a short interaction is observed between the ruthenium(II) center and a hydrogen atom on the ortho position of one of the nine phenyl rings.[4] Complexes of borohydride are described as using the three-center two-electron bonding model.
The nature of the interaction was foreshadowed in main group chemistry in the structural chemistry of trimethylaluminium.
Characteristics of agostic bondsEdit
Agostic interactions are best demonstrated by crystallography. Neutron diffraction data have shown that C−H and M┄H bond distances are 5-20% longer than expected for isolated metal hydride and hydrocarbons. The distance between the metal and the hydrogen is typically 1.8–2.3 Å, and the M┄H−C angle is in the range of 90°–140°. The presence of a 1H NMR signal that is shifted upfield from that of a normal aryl or alkane, often to the region normally assigned to hydride ligands. The coupling constant 1JCH is typically lowered to 70–100 Hz versus the 125 Hz expected for a normal sp3 carbon–hydrogen bond.
Structure of (C2H5)TiCl3(dmpe), highlighting an agostic interaction between the methyl group and the Ti(IV) center.[5]
Strength of bondEdit
On the basis of experimental and computational studies, the stabilization arising from an agostic interaction is estimated to be 10–15 kcal/mol. Recent calculations using compliance constants point to a weaker stabilisation (<10 kcal/mol).[6] Thus, agostic interactions are stronger than most hydrogen bonds. Agostic bonds sometimes play a role in catalysis by increasing 'rigidity' in transition states. For instance, in Ziegler–Natta catalysis the highly electrophilic metal center has agostic interactions with the growing polymer chain. This increased rigidity influences the stereoselectivity of the polymerization process.
Related bonding interactionsEdit
A sigma complex derived from (MeC5H4)Mn(CO)3 and triphenylsilane.[7]
The term agostic is reserved to describe two-electron, three-center bonding interactions between carbon, hydrogen, and a metal. Two-electron three-center bonding is clearly implicated in the complexation of H2, e.g., in W(CO)3(PCy3)2H2, which is closely related to the agostic complex shown in the figure.[8]Silane binds to metal centers often via agostic-like, three-centered Si┄H−M interactions. Because these interactions do not include carbon, however, they are not classified as agostic.
Anagostic bondsEdit
Certain M┄H−C interactions are not classified as agostic but are described by the term anagostic. Anagostic interactions are more electrostatic in character. In terms of structures of anagostic interactions, the M┄H distances and M┄H−C angles fall into the ranges 2.3–2.9 Å and 110°–170°, respectively.[2][9]
^Brookhart, Maurice; Green, Malcolm L. H.; Parkin, Gerard (2007). "Agostic interactions in transition metal compounds". Proc. Natl. Acad. Sci. 104 (17): 6908–14. Bibcode:2007PNAS..104.6908B. doi:10.1073/pnas.0610747104. PMC1855361. PMID 17442749.
^La Placa, Sam J.; Ibers, James A. (1965). "A Five-Coordinated d6 Complex: Structure of Dichlorotris(triphenylphosphine)ruthenium(II)". Inorg. Chem. 4 (6): 778–783. doi:10.1021/ic50028a002.
^Z. Dawoodi; M. L. H. Green; V. S. B. Mtetwa; K. Prout; A. J. Schultz; J. M. Williams; T. F. Koetzle (1986). "Evidence for Carbon–Hydrogen–Titanium Interactions: Synthesis and Crystal Structures of the Agostic alkyls [TiCl3(Me2PCH2CH2PMe2)R] (R = Et or Me)". J. Chem. Soc., Dalton Trans. (8): 1629. doi:10.1039/dt9860001629.
^Von Frantzius, Gerd; Streubel, Rainer; Brandhorst, Kai; Grunenberg, Jörg (2006). "How Strong is an Agostic Bond? Direct Assessment of Agostic Interactions Using the Generalized Compliance Matrix". Organometallics. 25 (1): 118–121. doi:10.1021/om050489a.
^Nikonov, G. I. (2005). "Recent Advances in Nonclassical Interligand SiH Interactions". Adv. Organomet. Chem. Advances in Organometallic Chemistry. 53: 217–309. doi:10.1016/s0065-3055(05)53006-5. ISBN9780120311538.
^Kubas, G. J. (2001). Metal Dihydrogen and σ-Bond Complexes. New York: Kluwer Academic. ISBN978-0-306-46465-2.
^Braga, D.; Grepioni, F.; Tedesco, E.; Biradha, K.; Desiraju, G. R. (1997). "Hydrogen Bonding in Organometallic Crystals. 6. X−H┄M Hydrogen Bonds and M┄(H−X) Pseudo-Agostic Bonds". Organometallics. 16 (9): 1846–1856. doi:10.1021/om9608364.
External linksEdit
Agostic interactions
August 31, 2023
agostic, interaction, organometallic, chemistry, agostic, interaction, refers, interaction, coordinatively, unsaturated, transition, metal, with, bond, when, electrons, involved, bond, enter, empty, orbital, transition, metal, resulting, three, center, electro. In organometallic chemistry agostic interaction refers to the interaction of a coordinatively unsaturated transition metal with a C H bond when the two electrons involved in the C H bond enter the empty d orbital of the transition metal resulting in a three center two electron bond 1 Many catalytic transformations e g oxidative addition and reductive elimination are proposed to proceed via intermediates featuring agostic interactions Agostic interactions are observed throughout organometallic chemistry in alkyl alkylidene and polyenyl ligands Contents 1 History 2 Characteristics of agostic bonds 2 1 Strength of bond 3 Related bonding interactions 3 1 Anagostic bonds 4 Function 5 References 6 External linksHistory EditThe term agostic derived from the Ancient Greek word for to hold close to oneself was coined by Maurice Brookhart and Malcolm Green on the suggestion of the classicist Jasper Griffin to describe this and many other interactions between a transition metal and a C H bond Often such agostic interactions involve alkyl or aryl groups that are held close to the metal center through an additional s bond 2 3 Short interactions between hydrocarbon substituents and coordinatively unsaturated metal complexes have been noted since the 1960s For example in tris triphenylphosphine ruthenium dichloride a short interaction is observed between the ruthenium II center and a hydrogen atom on the ortho position of one of the nine phenyl rings 4 Complexes of borohydride are described as using the three center two electron bonding model Mo PCy3 2 CO 3 featuring an agostic interactionThe nature of the interaction was foreshadowed in main group chemistry in the structural chemistry of trimethylaluminium Characteristics of agostic bonds EditAgostic interactions are best demonstrated by crystallography Neutron diffraction data have shown that C H and M H bond distances are 5 20 longer than expected for isolated metal hydride and hydrocarbons The distance between the metal and the hydrogen is typically 1 8 2 3 A and the M H C angle is in the range of 90 140 The presence of a 1H NMR signal that is shifted upfield from that of a normal aryl or alkane often to the region normally assigned to hydride ligands The coupling constant 1JCH is typically lowered to 70 100 Hz versus the 125 Hz expected for a normal sp3 carbon hydrogen bond Structure of C2H5 TiCl3 dmpe highlighting an agostic interaction between the methyl group and the Ti IV center 5 Strength of bond Edit On the basis of experimental and computational studies the stabilization arising from an agostic interaction is estimated to be 10 15 kcal mol Recent calculations using compliance constants point to a weaker stabilisation lt 10 kcal mol 6 Thus agostic interactions are stronger than most hydrogen bonds Agostic bonds sometimes play a role in catalysis by increasing rigidity in transition states For instance in Ziegler Natta catalysis the highly electrophilic metal center has agostic interactions with the growing polymer chain This increased rigidity influences the stereoselectivity of the polymerization process Related bonding interactions Edit A sigma complex derived from MeC5H4 Mn CO 3 and triphenylsilane 7 The term agostic is reserved to describe two electron three center bonding interactions between carbon hydrogen and a metal Two electron three center bonding is clearly implicated in the complexation of H2 e g in W CO 3 PCy3 2H2 which is closely related to the agostic complex shown in the figure 8 Silane binds to metal centers often via agostic like three centered Si H M interactions Because these interactions do not include carbon however they are not classified as agostic Anagostic bonds Edit Certain M H C interactions are not classified as agostic but are described by the term anagostic Anagostic interactions are more electrostatic in character In terms of structures of anagostic interactions the M H distances and M H C angles fall into the ranges 2 3 2 9 A and 110 170 respectively 2 9 Function EditAgostic interactions serve a key function in alkene polymerization and stereochemistry as well as migratory insertion References Edit IUPAC Compendium of Chemical Terminology 2nd ed the Gold Book 1997 Online corrected version 2006 agostic interaction doi 10 1351 goldbook AT06984 a b Brookhart Maurice Green Malcolm L H 1983 Carbon hydrogen transition metal bonds J Organomet Chem 250 395 408 doi 10 1016 0022 328X 83 85065 7 Brookhart Maurice Green Malcolm L H Parkin Gerard 2007 Agostic interactions in transition metal compounds Proc Natl Acad Sci 104 17 6908 14 Bibcode 2007PNAS 104 6908B doi 10 1073 pnas 0610747104 PMC 1855361 PMID 17442749 La Placa Sam J Ibers James A 1965 A Five Coordinated d6 Complex Structure of Dichlorotris triphenylphosphine ruthenium II Inorg Chem 4 6 778 783 doi 10 1021 ic50028a002 Z Dawoodi M L H Green V S B Mtetwa K Prout A J Schultz J M Williams T F Koetzle 1986 Evidence for Carbon Hydrogen Titanium Interactions Synthesis and Crystal Structures of the Agostic alkyls TiCl3 Me2PCH2CH2PMe2 R R Et or Me J Chem Soc Dalton Trans 8 1629 doi 10 1039 dt9860001629 Von Frantzius Gerd Streubel Rainer Brandhorst Kai Grunenberg Jorg 2006 How Strong is an Agostic Bond Direct Assessment of Agostic Interactions Using the Generalized Compliance Matrix Organometallics 25 1 118 121 doi 10 1021 om050489a Nikonov G I 2005 Recent Advances in Nonclassical Interligand SiH Interactions Adv Organomet Chem Advances in Organometallic Chemistry 53 217 309 doi 10 1016 s0065 3055 05 53006 5 ISBN 9780120311538 Kubas G J 2001 Metal Dihydrogen and s Bond Complexes New York Kluwer Academic ISBN 978 0 306 46465 2 Braga D Grepioni F Tedesco E Biradha K Desiraju G R 1997 Hydrogen Bonding in Organometallic Crystals 6 X H M Hydrogen Bonds and M H X Pseudo Agostic Bonds Organometallics 16 9 1846 1856 doi 10 1021 om9608364 External links EditAgostic interactions Retrieved from https en wikipedia org w index php title Agostic interaction amp oldid 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