The pincer-ligated species (PCP)Ir (PCP = kappa3-C6H3-2,6-(CH2PtBu2)2) is found to promote dimerization of phenylacetylene to give the enyne complex (PCP)Ir(trans-1,4-phenyl-but-3-ene-1-yne). The mechanism of this reaction is found to proceed through three steps: (i) addition of the alkynyl C-H bond to iridium, (ii) insertion of a second phenylacetylene molecule into the resulting Ir-H bond, and (iii) vinyl-acetylide reductive elimination. Each of these steps has been investigated, by both experimental and computational (DFT) methods, to yield unexpected conclusions of general interest. (i) The product of alkynyl C-H addition, (PCP)Ir(CCPh)(H) (3), has been isolated and, in accord with experimental observations, is calculated to be 29 kcal/mol more stable than the analogous product of benzene C-H addition. (ii) Insertion of a second PhCCH molecule into the Ir-H bond of 3 proceeds rapidly, but with a 1,2-orientation. This orientation gives (PCP)Ir(CCPh)(CPh=CH2) (4) which would yield the 1,3-diphenyl-enyne if it were to undergo C-C elimination; however, the insertion is reversible, which represents the first example, to our knowledge, of simple beta-H elimination from a vinyl group to give a terminal hydride. The 2,1-insertion product (PCP)Ir(CCPh)(CH=CHPh) (6) forms more slowly, but unlike the 1,2 insertion product it undergoes C-C elimination to give the observed enyne. (iii) The failure of 4 to undergo C-C elimination is found to be general for (PCP)Ir(CCPh)(vinyl) complexes in which the vinyl group has an alpha-substituent. Thus, although C-C elimination relieves crowding, the reaction is inhibited by increased crowding. Density-functional theory (DFT) calculations support this surprising conclusion and offer a clear explanation. Alkynyl-vinyl bond formation in the C-C elimination transition state involves the vinyl group pi-system; this requires that the vinyl group must rotate (around the Ir-C bond) by ca. 90 degrees to achieve an appropriate orientation. This rotation is severely inhibited by steric crowding, particularly when the vinyl group bears an alpha-substituent.

The reductive elimination of carbon-carbon bonds is one of the most fundamentally and synthetically important reaction steps in organometallic chemistry, yet relatively little is understood about the factors that govern the kinetics of this reaction. C-C elimination from complexes with the common d (6) six-coordinate configuration generally proceeds via prior ligand loss, which greatly complicates any attempt to directly measure the rates of the specific elimination step. We report the synthesis of a series of five-coordinate d (6) iridium complexes, ( (tBu)PCP)Ir(R)(R'), where R and R' are Me, Ph, and (phenyl-substituted) vinyl and alkynyl groups. For several of these complexes (R/R' = Ph/Vi, Me/Me, Me/Vi, Me/CCPh, and Vi/CCPh, where Vi = trans-CHCHPh) we have measured the absolute rate of C-C elimination. For R/R' = Ph/Ph, Ph/Me, and Ph/CCPh, we obtain upper limits to the elimination rate; and for R/R' = CCPh/CCPh, a lower limit. In general, the rates decrease (activation barriers increase) according to the following order: acetylide < vinyl approximately Me < Ph. Density functional theory (DFT) calculations offer significant insight into the factors behind this order, in particular the slow rates for elimination of the vinyl and, especially, phenyl complexes. The transition states are calculated to involve rotation of the aryl or vinyl group around the Ir-C bond, prior to C-C elimination, such that the group to which it couples can add to the face of the aryl or vinyl group. This rotation is severely hindered by the presence of the phosphino -t-butyl groups that lie above and below the plane of the aryl/vinyl group in the ground state. Accordingly, calculations predict dramatically different relative rates of elimination from the much less sterically hindered complexes ( (H)PCP)Ir(R)(R'). For example, the barrier to elimination from ( (H)PCP)Ir(Me) 2 is 20 kcal/mol, which is 2 kcal/mol greater than from the ( (tBu)PCP)Ir analogue. In contrast, the activation enthalpies calculated for vinyl-vinyl and phenyl-phenyl elimination from ( (H)PCP)Ir are remarkably low, only 2 and 9 kcal/mol, respectively; these values are 16 and 22 kcal/mol less than those of the corresponding ( (tBu)PCP)Ir complexes. Moreover, since these eliminations are very nearly thermoneutral, the barriers are calculated to be equally low for the reverse reactions [C-C oxidative addition to ( (H)PCP)Ir]. The absence of differences in intraligand CC bond lengths in the transition states relative to the ground states, combined with a comparison of calculated "face-on" and "planar" transition states for C-C coupling, suggests that the critical importance of the aryl/vinyl rotation is based on geometric or steric factors rather than electronic ones. Thus there is no evidence for participation of the pi or pi* orbitals of the aryl or vinyl groups in the formation of the C-C bond, although a small pi effect cannot be rigorously excluded. Likewise, the results do not support the hypothesis that the degree of directionality of the carbon-based orbital u...

The reaction of [IrCl(COE)2]2 (1, COE = cyclooctene) with pincer ligand HN(CH2CH2P i Pr2)2 ((PNP) H ) and AgPF6 gives iridium(I) amino olefin complex [Ir(COE)(PNP) H ]PF6 (3 COE -PF 6 ). Without anion exchange, the stability of 3 COE -Cl is highly solvent dependent. In benzene or THF a mixture of amido complex [Ir(COE)(PNP)] (4 COE ), [IrHCl2(PNP) H ] (5), and [IrHCl(C8H13)(PNP) H ] (6) with a vinylic cyclooctenyl ligand is obtained. A pathway is proposed that includes concurrent trapping of intramolecular C−H versus intermolecular N−H activation products. 3 L -PF 6 (L = C2H4, C3H6, CO) are prepared by olefin substitution. Deprotonation with KOtBu gives the corresponding amido complexes [IrL(PNP)] (4 L ; L = COE, C2H4, CO). Reversible COE C−H activation is proposed to account for the fluxional behavior of 3 COE -PF 6 in solution as compared with the structural rigidity of 4 COE , which points toward strong N→Ir π-donation in the amido complexes.

Ir haVe been preViously reported to react with dinitrogen to giVe the bridging dinuclear complex [(PCP)Ir] 2 (N 2 ) (2), which was crystallographically characterized. We report that under N 2 atmosphere the only obserVable nitrogen complex in solution is actually the terminal dinitrogen complex (PCP)Ir(N 2 ). The mono-and dinuclear species are in equilibrium; the dinuclear complex more readily crystallizes from solution.

The reaction of nitromethane with (PCP)Ir (PCP = κ3-2,6-( t Bu2PCH2)2C6H3) yields the bidentate O,O-ligated nitromethanate complex (PCP)Ir(H)(κ2-O,O-NO2CH2) (1). Reaction of 1 with CO affords a CO adduct with a mono-oxygen-ligated nitromethanate, 2, which represents the first characterized transition metal mono-oxygen-ligated nitromethanate complex. At elevated temperature, complex 2 isomerizes to give the carbon-bound nitromethyl complex 3. Complex 1 also undergoes addition of cyclohexylisocyanide (analogous to the reaction with CO) to form the mono-oxygen-ligated nitromethanate complex 4, which also isomerizes to form the corresponding nitromethyl complex, 5. The (PCP)Ir-(CH3NO2) system is the first species known to display three binding modes with a nitromethanate anion. Results from density functional calculations illustrate the structures and energies of the minima and transition states on the potential energy surfaces. The calculations suggest that 1 is the thermodynamic product of (PCP)Ir reacting with nitromethane; a kinetic product, formed via oxidative addition of a nitromethane C−H bond, should readily rearrange to form 1.