Metal Hydrides, Alkyls, Aryls, and their Reactions
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1 Metal Hydrides, Alkyls, Aryls, and their Reactions
2
3 A Primer on MO Theory
4
5
6
7 σ-bonding in Organotransition Metal Complexes
8
9 M-C Bond Energies in Organotransition Metal Complexes
10 Thermodynamic Predictions
11 σ-bonding Ligands The σ-binding ligands are hydride (H - ), alkyl (R - ), and aryl (Ar - ) I. Hydride Ligands H anionic 2e donor { M H bridging mode M M H 1e to each M
12 Structure of hydride complexes
13
14 Synthesis of hydride complexes
15 Characterization
16 Acidity HCo(CO) 4 1 H NMR = 10.7 ppm HCo(CO) 4 H + + Co(CO) 4 strong acid in H 2 O, MeOH, similar to HCl!! d 0 Cp* 2 ZrH 2 δ = ppm d 10 [HCu{P(p-tolyl) 3 }] 6 δ = ppm The presence of partially filled d orbitals on the transition metal has a considerable shielding effect (moves the chemical shift of the NMR resonance to more negative ppm) on the hydride 1H NMR chemical shift position. Late Transition Metals: tend to be more protic Early Transition Metals: tend to be more hydridic
17
18 pka Values for Transition Metal Hydrides in Various Solvents Solvent M-H Complex H 2 O MeOH CH 3 CN HV(CO) 6 strong acid HV(CO) 5 (PPh 3 ) 6.8 CpCrH(CO) CpMoH(CO) Cp*MoH(CO) CpWH(CO) CpWH(CO) 2 (PMe 3 ) 26.6
19 Solvent M-H Complex H 2 O MeOH CH 3 CN HMn(CO) HRe(CO) 5 ~21 H 2 Fe(CO) H 2 Ru(CO) H 2 Os(CO) CpFeH(CO) Cp*FeH(CO) CpRuH(CO)
20 Solvent M-H Complex H 2 O MeOH CH 3 CN HCo(CO) 4 strong acid strong acid HCo(CO) 3 {P(OPh) 3 } HCo(CO) 3 (PPh 3 ) HNi[P(OMe) 3 ] HPd[P(OMe) 3 ] HPt[P(OMe) 3 ] H 4 Ru 4 (CO) H 4 Os 4 (CO) H 2 Ru 4 (CO)
21 Hydrogenation of unsaturated compounds Olefin hydrogenation by Pd/C is one of the oldest metal catalyzed reactions Homogeneous catalytic cycle and the mechanistic details carefully worked out Three steps involved in any hydrogenation reaction: (a) Hydrogen activation: oxidative addition, homolytic, or heterolytic (b) Migratory insertion (c) Alkane elimination: reductive elimination, σ-bond metathesis, or homolytically
22 Olefin hydrogenation using Wilkinson s catalyst, RhCl(PPh 3 ) 3 * Rh complexes outside the box have actually been observed under the catalytic conditions * Complexes outside the box have not been observed
23 II. Alkyl and Aryl Ligands M C(alkyl) σ-bonding M C(aryl) σ-bonding dz 2 sp 3 dz 2 sp 2 Alkyls - typically very strong anionic σ-donors, second only to hydrides; they have virtually no π-acceptor ability Increasing the carbon substitution (methyl ethyl isopropyl) usually increases the donor strength, but steric factors can come into play and weaken the metal-alkyl bond (e.g., t-butyl groups are often too sterically hindered to bind well) Replacing the hydrogens with fluorine atoms (very electron withdrawing) dramatically reduces the donor ability of the alkyl (aryl). For example, CF 3 and C 6 F 5 are not very strong donors
24 Synthesis of metal alkyls 1. Alkylation with a nucleophilic organometallic species 2. Nucleophilic addition to an electrophilic alkyl J. Am. Chem. Soc. 1983, 105, Oxidative addition 4. Insertion
25 Metal alkyls are extremely reactive toward O 2, water, and a variety of other ligands and reagents As with hydrides, they play a very important and active role in catalysis As the metal electronegativity increases, the nucleophilic reactivity of the M-C bond decreases Nucleophilic reactivity decreases with more stable carbanions (i.e., CH 3- < C 6 H 5- < CCR - Early transition metal alkyls are very sensitive toward oxygen and water Late transition metal alkyls are more stable toward oxygen and water
26 Stability of transition metal alkyls The M-C bond was thought to be quite weak to give stable products Actually the lack of kinetic stability rather than the thermodynamic stability is the reason for the lack of stability of the transition metal alkyl complexes Decomposition pathways (ι) β-hydride elimination (ιι) α-elimination (iii) reductive elimination
27 (ι) β-hydride elimination -most common decomposition pathway available for metal alkyls -occurs for nearly all metal alkyl complexes that have the following features: a. β-carbon must have hydrogen substituents b. the M C C H fragment can adopt a roughly coplanar conformation that brings the β-hydrogen close to the metal center c. a vacant coordination site (orbital) cis to the alkyl substituent
28 β-hydride elimination empty orbital M H C H α carbon M H H H C β hydrogen H H β carbon H H H
29 Main driving force for β-hydride elimination - formation of a stronger M-H bond (almost always stronger than M-alkyl) and the generation of an alkene ligand that reduces the unsaturation of the metal complex The reverse reaction, however, also can occur and is called a migratory insertion In order to have a β-hydride elimination you MUST have an empty orbital on the metal cisoidal (next) to the alkyl ligand. You also must have β-hydrogens present on the alkyl In order to prepare stable M-alkyl complexes one, therefore, often needs to stay away from alkyls with β-hydrogens (or avoid metals with empty coordination sites)
30 Some common ligands used to avoid β-hydride elimination reactions: M M CH 3 Me Me M M Si Me Me Me methyl neopentyl benzyl trimethylsilylmethyl Me alkyls that do not have β-hydrogens alkyls for which the β-hydrogens cannot approach the metal center alkyls in which the M C C H fragment cannot become coplanar 18 electron species that cannot undergo ligand dissociation some d 0 alkyls
31 (ii) α-elimination -less common reaction with metal alkyls is the α-hydride elimination -a hydrogen atom on the α-carbon is added to the metal to generate a M=CR 2 (carbene or alkylidene) group and a hydride -for early transition metal alkyls, α-elimination can be an important decomposition pathway
32 Me Me P P Me Ta Cl H H Cl α-hydride elmination could also be considered an oxidative addition Me Me Cl P P Me Ta Cl H H migratory insertion Me Me P P Me Ta Cl Cl Note that just as with a β-hydride elimination, it is important to have an empty orbital cis to the α-hydrogen in order to have the α-hydride elimination occur
33 (iii) Reductive elimination
34 Aryl ligands relatively strong anionic two electron donors, essentially just like alkyls Since they cannot easily β-hydride eliminate (formation of the benzyne intermediate is typically too unstable), metal aryl complexes are usually relatively stable compared to alkyls with β-hydrogens do have the potential for both π-donation and π-backbonding through the filled aryl π-orbitals and empty π* antibonding orbitals This can provide additional stability to a metal complex, depending on whether the metal needs additional electrons from the ligand or wants to dump excess electron density onto the ligand M
35 Trends in M-H and M-Me bond strengths M-Me bonds are NOT weak! * e.g., Ta-Me 260 kj/mol Ge-Me 240 kj/mol * high reactivity is kinetic in origin M-H bonds are stronger than M-Me bonds for a given metal * e.g., Zr-Me 285 kj/mol Zr-H 339 kj/mol Rationale: better overlap with the spherical 1s orbital, no non-bonding electron repulsions and minimal steric repulsion for H
36 M-Me bond strength increases down a GROUP e.g., Mn-Me 187 kj/mol Re-Me 220 kj/mol Rationale: overlap between the C 2s and 2p hybrids and TM d orbitals improves with increasing principal quantum no (3d are too contracted, projection of valence d orbitals beyond filled s and p shells is greater for 2 nd and 3 rd row TMs) Note: this trend is the opposite of that observed in the main group; Si-Me 290 kj/mol Pb-Me 130 kj/mol
37 M-Me bond strength increases to the left in the d-block (early metals form stronger bonds to carbon) e.g., Ti-Bz 201 kj/mol Co-Bz 120 kj/mol Rationale: better radial extension for the d-orbitals and a better energy match with the C 2s and 2p orbitals (the latter reaches a maximum for groups 6-8)
38 The difference between M-H and M-Me ( Me/H ) bond strengths increases from left to right across the d-block Rationale: late d-block metals (groups 9-10) possess more nonbonding electrons than do early metals and this results in greatly increased repulsions with alkyl non-bonding electrons (in C-H or C-C bonds); H has no such electrons so this effect does not apply It should be noted that electron density in C-H bonds is known to stabilize early transition metal alkyls (agostic interaction)
39 σ-complexes involving dihydrogen, silanes, boranes, alkanes (review: Crabtree, Angew. Chem. Int. Ed. Engl. 1993, 32, 789) (a) Dihydrogen complexes: first reported in 1984 by Kubas et al. (JACS, 1984, 96, 480) (b) Alkane complexes (i) Agostic C-H complexes: coordination of a C-H bond that is present in a bound ligand (original description: Brookhart and Green, J. Organomet. Chem. 1983, 350, 395)
40 Characterization of agostic complexes Crystallography: characterized by close M-H contacts C-H bonds are lengthened; M-H distance is longer than for a metal hydride NMR: both 1 H and 13 C appear upfield compared to normal alkyl chemical shifts J( 13 C- 1 H) is typically lower than for a non-bridged species ( Hz vs Hz) agostic interactions are often fluxional isotopic perturbation - powerful method for identifying agostic interactions IR: lower ν(ch) are sometimes observed ( cm -1 ) Agostic interactions - stabilize MR; considered as stable intermediates in insertion/β-hydride elimination; role in olefin polymerization catalysts
41 (ii) Alkane complexes Alkanes can act as ligands to metal centers, but bind very weakly direct observation of alkane complexes is difficult alkane complexes are thought to be key intermediates in oxidative addition/reductive elimination of alkanes Alkane binding modes:
42 Observation by time-resolved IR many examples of 16 e metal carbonyls coordinating to alkanes have been observed, although lifetimes of these complexes are very short (300 ns to 200 µs) Observation by NMR: JACS, 1998, 120, 9953
43 Crystallographic characterization??: JACS, 1997, 119, 3633
44 (c) Silanes and boranes
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