Activation of C—H Bonds in Ligands

Evidence of the activation of C—H bonds in ligands by the transition metal to which the ligand is attached has come from three sources. 

Hydrogen-deuterium exchange in phosphine ligands. Here hydrogen in the phosphine is exchanged for deuterium that is either in the 

Compounds with hydrogen in a ligand coordinated to a transition metal. Here the hydrogen in the ligand occupies one of the coordination sites around the metal. 

Intramolecular Ring-Closure Reactions of Phosphine Ligands 

The first reported interaction between a C—H bond in a ligand and a soluble transition metal complex was that reported by Chatt and Davidson (5). The reaction product from the reaction of sodium naphthalene with (rans-dichlorodi-(dmpe)ruthenium(II) (I) (dmpe = 1,2-bisdimethylphosphinoethane) was found to be a tautomeric equilibrium mixture of the ruthenium(O) complex (II) that contains a 7r-complexed naphthalene ligand and the ruthenium(II)-hydrido complex (III) where a C(2)—H bond has added to the ruthenium  

---

The intramolecular activation of C-H bonds in ligands coordinated onto platinum, has also received much attention (37) the formation of five-mem-bered rings is the most favorable and may explain the observed regioselectivities ... 

These findings have stimulated enormously the search for intermolecular activation of C-H bonds, in particular those of unsubstituted arenes and alkanes. In 1982 Bergman [2] and Graham [3] reported on the reaction of well-defined complexes with alkanes and arenes in a controlled manner. It was realised that the oxidative addition of alkanes to electron-rich metal complexes could be thermodynamically forbidden as the loss of a ligand and rupture of the C-H bond might be as much as 480 kl.mol, and the gain in M-H and M-C... 

Note Activation of C-H bonds in the N-sidechains of NHC ligands is a frequently occurring phenomenon. 

Again, the methylidene complex shows different behavior than the other carbenes. The decomposition of the methylidene is not affected by the presence of excess phosphine and is first order in catalyst. The decomposition in this case appears to be due to activation of C-H bonds in the L ligand. 

Complexes of the group 9 metals, especially rhodium and iridium, represent one of the more numerous families on which a systematic appraisal of struc-ture/bonding properties for the boryl ligand can be based. In part, this reflects the involvement of such systems not only in earlier work on metal-catalyzed hydroboration chemistry [2-5,35,113-123], but in more recent studies of di-boration [124,125], and the activation of C-H bonds in both saturated [9,10, 126-135] and unsaturated hydrocarbons [9,10,50,51,127,129,134,136-159]. 

The authors established directly the time scale for activation of C-H bonds in solutions at room temperature by monitoring the C-H bond activation reaction in the nanosecond regime with infrared detection. In the first stage of the process, loss of one carbon monoxide ligand (reaction VI-7 —- VI-8 in Scheme VI.6) substantially reduces back-bonding from the rhodium ion and increases the electron density at the metal center. Formed after the solvation stage, complex VI-9 traverses a 4.2 kcai nriol barrier (A = 5.0 x lo s ) and forms the -pCTp complex VI-10 which is more reactive toward C-H oxidative addition. 

Figure 6 Activation of C-H bonds in aldehydes by an Mdium complex within a tetrahedral metal-organic host in aqueous solution, (a) Crystal structure of the host, viewed down the C2-axis of S5munetry (b) simplified view, showing the structure of one of the six identical ligands that comprise the edges of the tetrahedral host (c) schematic representation of host and (d) proposed mechanism for C-H bond activation by an encapsulated Ir complex.

Activation of C-H bonds in hydrocarbons, a-BOND METATHESIS AND H2 AS A LIGAND... 

Facile C-H bond activation by Pt(II) metal centers seems to require at least one labile ligand in the coordination sphere of platinum. One of the earliest intermolecular examples of this is the activation of C-D bonds in benzene-f/, by 0 an.S -(PAIe .) Pt(neopentyl)(OTf) at 133 °C, where trifluoromethanesulfonate (triflate, OTf) provides the labile group (Scheme 7, A) (26). 

Example Selective activation of C-H bonds is rarely observed in saturated alkyl groups, but the iridium complex 1 does react by C-H insertion of the metal into a ligand bond upon treatment with LiBr in solution. The reaction can be tracked by LT-FAB-MS (Fig. 9.17). A decreasing intensity of the molecular ion of 1, m/z 812.4, and increasing of 2, m/z 856.4, indicate the progress of this reaction. Furthermore, the halogen exchange is indicated by the isotopic pattern. 

In connection with the activation of C—H bonds the cyclometalation has become a very general reaction, intensively investigated by Kaesz and coworkers4 and reviewed by Bruce.5 In transition metal complexes an organic ligand reacts with the transition metal resulting in the formation of a metal-... 

A second, very important decomposition pathway involves the activation of C-H bonds on the N-alkyl [142-147] or N-aryl [148-151] sidechains. OccassionaUy, even C-C activation in the sidechain is observed [152]. Similar C-H activation is observed in transition metal phosphides, especially when the phosphorus ligand has a SMes substituent [153]. 

The activation of C-H bonds by an electrophilic pathway is shown schematically in eq. (12) and has been observed with a number of late transition metal ions [9], A driving force for the reaction shown in eq. (12) is the stabilization of the leaving group, H", by solvation in polar solvents. The related four-center electrophilic activation by transition, lanthanide, and actinide metal centers has also been reported, (eqs. (13a) and (13b)) [9b,c,g, 27]. In these instances, a ligand on the metal assists the reaction by acting as the base. 

The effects of ligands in the activation of C-H bonds by transition metal complexes were discussed in certain works. It has been suggested [66a] that a combination of hydride and lone-pair ligands with a minimum of 7c-bonding should be an optimal combination for the reaction between methane and some model Rh(I) and Ru(II) complexes. For example, it should be advantageous to... 

Beginning with B, the C-H activation by bpym system favors electrophilic substitution (T2) over oxidative addition (T2b), distinguishing from the diamine system where oxidative addition is more favorable than electrophilic substitution. This indicated that C-H activation could involve either electrophilic substitution or oxidative addition depending upon the ligands [77]. Interestingly, activation of C-H bond by Pt(II) systems was found to be facile, whereas the most difficult step in overall reaction process is methane uptake (step (1)). Recent calculations by Zielger s group also supported this point [80, 81]. 

For this triad, only iron (Table 4.17) gives homoleptic complexes containing (tM — C bonds. The remaining elements form heteroleptic compounds. The stability of phosphine complexes of the type M(aryl)2 (PR3)2 decreases considerably according to the series Ni>Co>Fe. Ruthenium and osmium complexes characteristically activate the C —H bond in coordinated ligands and form compounds with (tM—C bonds as a result of oxidative addition. The following reactions serve as examples ... 

---