ion. The oxidative addition mechanism was originally proposed22i because of the lack of a strong rate dependence on polar factors and on the acidity of the medium. Later, however, the electrophilic substitution mechanism also was proposed. Recently, the oxidative addition mechanism was confirmed by investigations into the decomposition and protonolysis of alkylplatinum complexes, which are the reverse of alkane activation. There are two routes which operate in the decomposition of the dimethylplatinum(IV) complex Cs2Pt(CH3)2Cl4. The first route leads to chloride-induced reductive elimination and produces methyl chloride and methane. The second route leads to the formation of ethane. There is strong kinetic evidence that the ethane is produced by the decomposition of an ethylhydridoplatinum(IV) complex formed from the initial dimethylplatinum(IV) complex. In D2O-DCl, the ethane which is formed contains several D atoms and has practically the same multiple exchange parameter and distribution as does an ethane which has undergone platinum(II)-catalyzed H-D exchange with D2O. Moreover, ethyl chloride is formed competitively with H-D exchange in the presence of platinum(IV). From the principle of microscopic reversibility it follows that the same ethylhydridoplatinum(IV) complex is the intermediate in the reaction of ethane with platinum(II). Important results were obtained by Labinger and Bercaw62c in the investigation of the protonolysis mechanism of several alkylplatinum(II) complexes at low temperatures. These reactions are important because they could model the microscopic reverse of C-H activation by platinum(II) complexes. Alkylhydridoplatinum(IV) complexes were observed as intermediates in certain cases, such as when the complex (tmeda)Pt(CH2Ph)Cl or (tmeda)PtMe2 (tmeda ) N,N,N′,N′-tetramethylenediamine) was treated with HCl in CD2Cl2 or CD3OD, respectively. In some cases H-D exchange took place between the methyl groups on platinum and the, CD3OD prior to methane loss. On the basis of the kinetic results, a common mechanism was proposed to operate in all the reactions: (1) protonation of Pt(II) to generate an alkylhydridoplatinum(IV) intermediate, (2) dissociation of solvent or chloride to generate a cationic, fivecoordinate platinum(IV) species, (3) reductive C-H bond formation, producing a platinum(II) alkane σ-complex, and (4) loss of the alkane either through an associative or dissociative substitution pathway. These results implicate the presence of both alkane σ-complexes and alkylhydridoplatinum(IV) complexes as intermediates in the Pt(II)-induced C-H activation reactions. Thus, the first step in the alkane activation reaction is formation of a σ-complex with the alkane, which then undergoes oxidative addition to produce an alkylhydrido complex. Reversible interconversion of these intermediates, together with reversible deprotonation of the alkylhydridoplatinum(IV) complexes, leads to multiple H-D exchange

Activation of c-h bonds by metal complexes

The selective transformation of ubiquitous but inert C–H bonds to other functional groups has far-reaching practical implications, ranging from more efficient strategies for fine chemical synthesis to the replacement of current petrochemical feedstocks by less expensive and more readily available alkanes. The past twenty years have seen many examples of C–H bond activation at transition-metal centres, often under remarkably mild conditions and with high selectivity. Although profitable practical applications have not yet been developed, our understanding of how these organometallic reactions occur, and what their inherent advantages and limitations for practical alkane conversion are, has progressed considerably. In fact, the recent development of promising catalytic systems highlights the potential of organometallic chemistry for useful C–H bond activation strategies that will ultimately allow us to exploit Earth's alkane resources more efficiently and cleanly.

Understanding and exploiting C–H bond activation

One {open_quotes}Holy Grail{close_quotes} of C-H activation research, therefore, is not simply to find new C-H activation reactions but to obtain an understanding of them that will allow the development of reagents capable of selective transformations of C-H bonds into more reactive functionalized molecules. Although a few interesting organic and main group reagents have been discovered that show intriguing potential for meeting this goal, much of the activity in this field has been in transition metal chemistry. The present Account therefore reflects this emphasis. 109 refs., 3 figs.

Selective Intermolecular Carbon-Hydrogen Bond Activation by Synthetic Metal Complexes in Homogeneous Solution

It is proposed that spin-crossing effects can dramatically affect reaction mechanisms, rate constants, branching ratios, and temperature behaviors of organometallic transformations. This phenomenon is termed two-state reactivity (TSR) and involves participation of spin inversion in the rate-determining step. While the present analysis is based on studies of transition metals under idealized conditions, several recent reports imply that TSR is by no means confined to the gas phase. In fact, participation of more than a single spin surface in the reaction pathways is proposed as a key feature in organometallic chemistry.

Two-State Reactivity as a New Concept in Organometallic Chemistry§

A bis(oxazoline) (box) copper(II) complex and its hydrated counterpart (1 and 2) function as enantioselective Lewis acid catalysts for carbocyclic and hetero Diels−Alder, aldol, Michael, ene, and amination reactions with substrates capable of chelation through six- and five-membered rings. X-ray crystallography of the chiral complexes reveals a propensity for the formation of distorted square planar or square pyramidal geometries. The sense of asymmetric induction is identical for all the processes catalyzed by [Cu((S,S)-t-Bu-box)](X)2 complexes 1 and 2 (X = OTf and SbF) resulting from the intervention of a distorted square planar catalyst-substrate binary complex. These catalyzed processes exhibit excellent temperature−selectivity profiles. Reactions catalyzed by [Cu(S,S-Ph-pybox)](SbF6)2 and their derived chelation complexes are also discussed.

Chiral bis(oxazoline) copper(II) complexes: versatile catalysts for enantioselective cycloaddition, Aldol, Michael, and carbonyl ene reactions.

Sequential bond energies of Fe(CO)x + (x = 1-5): Systematic effects on collision-induced dissociation measurements

The sequential bond energies of Cr(CO) x + , x=1-6, are determined by collision-induced dissociation in a guided ion beam tandem mass spectrometer. Values for the 0 K bond energies (in eV) are determined to be D(Cr + -CO)=0.93±0.04, D[(CO)Cr + -CO]=0.98±0.03, D[(CO) 2 Cr + -CO]=0.56±0.06, D[(CO) 3 Cr + -CO]=0.53±0.08, D[(CO) 4 Cr + -CO]=0.64±0.03, and D[(CO) 5 Cr + -CO]=1.35±0.08

Sequential bond energies of chromium carbonyls (Cr(CO)x+, x = 1-6)

Reactions of N+4 with rare gases from thermal to 10eV center-of-mass energy: collision-induced dissociation, charge transfer and ligand exchange

The photoinitiated reaction of Cp*Rh(CO)[sub 2] with neopentane and neopentane-d[sub 12] in liquid krypton has been studied with low temperature IR flash kinetic spectroscopy. Photolysis of Cp*Rh(CO)[sub 2] generates a single transient absorption at 1946 cm[sup [minus]1] which is assigned to the Cp*Rh(CO)(Kr) complex. This complex reacts with (CH[sub 3])[sub 4]C to form the C-H activated neopentyl hydride product observed at 2008 cm[sup [minus]1]. Confirming earlier flash kinetic results with cyclohexane, the results are consistent with a pre-equilibrium mechanism in which an initially formed transient krypton complex Cp*Rh(CO)(Kr) is in rapid equilibrium with a transient (uninserted) alkane complex Cp*Rh(CO)(CH[sub 3][sub 4]C) which then proceeds to form the neopentyl hydride in a unimolecular step. Under most conditions our mechanism requires that both Cp*Rh(CO)(Kr) and Cp*Rh(CO)(alkane) exhibit unresolved carbonyl stretching absorptions at 1946 cm[sup [minus]1]. 44 refs., 7 figs., 1 tab.

Activation of the C-H Bonds in Neopentane and Neopentane-d12 by (.eta.5-C5(CH3)5)Rh(CO)2: Spectroscopic and Temporal Resolution of Rhodium-Krypton and Rhodium-Alkane Complex Intermediates

Flash kinetic spectroscopy with infrared detection is used to prove C-H activation of cyclohexane-d[sub 0] and -d[sub 12] by intermediates generated upon ultraviolet irradiation of Cp*Rh(CO)[sub 2] (Cp* = C[sub 5](CH[sub 3])[sub 5]) in liquid rare gas (Rg = Kr or Xe) solution at low temperature (163-193 K). Upon UV photolysis, a new C-O stretching band (at 1946.5 cm[sup [minus]1] in Kr and at 1941.5 cm[sup [minus]1] in Xe) appears promptly, which we attribute to Cp*Rh(CO)(Rg). In the presence of hydrocarbon a second C-O stretching band (2002.5 cm[sup [minus]1] in Kr and 1998 cm[sup [minus]1] in Xe) grows in at the same observed rate as the disappearance of the 1946.5-cm[sup [minus]1] band. We attribute this second band to the alkyl hydride product, Cp*Rh(CO)(H)(C[sub 6]H[sub 11]). The concentration dependence of the observed reaction rate in Kr solution shows behavior consistent with a preequilibrium mechanism in which the initial Cp*Rh(CO)(Rg) complex equilibrates with a weakly bound Cp*Rh(CO)(alkane) complex (which has an IR carbonyl frequency unresolvable from that of the rare gas complex) followed by C-H activation of the latter. 53 refs., 13 figs., 1 tab.

IR Flash Kinetic Spectroscopy of C-H Bond Activation of Cyclohexane-d0 and -d12 by Cp*Rh(CO)2 in Liquid Rare Gases: Kinetics, Thermodynamics, and Unusual Isotope Effect

Richard H. Schultz

Papers

Sequential bond energies of Fe(CO)x + (x = 1-5): Systematic effects on collision-induced dissociation measurements

Sequential bond energies of chromium carbonyls (Cr(CO)x+, x = 1-6)

Reactions of N+4 with rare gases from thermal to 10eV center-of-mass energy: collision-induced dissociation, charge transfer and ligand exchange

Activation of the C-H Bonds in Neopentane and Neopentane-d12 by (.eta.5-C5(CH3)5)Rh(CO)2: Spectroscopic and Temporal Resolution of Rhodium-Krypton and Rhodium-Alkane Complex Intermediates

IR Flash Kinetic Spectroscopy of C-H Bond Activation of Cyclohexane-d0 and -d12 by Cp*Rh(CO)2 in Liquid Rare Gases: Kinetics, Thermodynamics, and Unusual Isotope Effect