Gregory J. Kubas

Metal–dihydrogen and σ-bond coordination: the consummate extension of the Dewar–Chatt–Duncanson model for metal–olefin π bonding

Abstract There is a marvelous analogy between the metal–olefin π bonding model first brought to light by Dewar 50 years ago and that of σ-bond coordination discovered by us 17 years ago. In some ways it is surprising that 33 years elapsed between the two parallel bonding situations. However this difference pales in comparison to that between the actual synthesis of the first olefin complex, Zeises salt in 1837, and the first recognized dihydrogen complex nearly 150 years later. This article delineates the principles of σ-bond coordination and activation inspired by the Dewar–Chatt–Duncanson model and illuminates the often-spectacular interplay between theory and experiment in this field. Aside from HH bond coordination and activation towards cleavage, the structure and bonding principles apply to SiH, CH, and virtually any two-electron XH or XY bond. Metal d to σ* XH backdonation is the key to stabilizing σ-bond coordination and is also crucial to homolytic cleavage (oxidation addition). There are some differences in bonding depending on X, and, in the case of BH bond coordination, in metal–borane complexes, backdonation to boron p orbitals occurs. For electrophilic complexes, particularly cationic systems with minimal backdonation, heterolytic cleavage of XH is common and is a key reaction in industrial and biological catalysis. Thus there are two separate pathways for σ-bond activation that directly depend on the electronics of the metal σ-ligand bonding.

Dihydrogen complexes as prototypes for the coordination chemistry of saturated molecules

The binding of a dihydrogen molecule (H2) to a transition metal center in an organometallic complex was a major discovery because it changed the way chemists think about the reactivity of molecules with chemically “inert” strong bonds such as HH and CH. Before the seminal finding of side-on bonded H2 in W(CO)3(PR3)2(H2), it was generally believed that H2 could not bind to another atom in stable fashion and would split into two separate H atoms to form a metal dihydride before undergoing chemical reaction. Metal-bound saturated molecules such as H2, silanes, and alkanes (σ-complexes) have a chemistry of their own, with surprisingly varied structures, bonding, and dynamics. H2 complexes are of increased relevance for H2 production and storage in the hydrogen economy of the future.

HETEROLYTIC SPLITTING OF HH, SiH, AND OTHER σ BONDS ON ELECTROPHILIC METAL CENTERS

Publisher Summary This chapter discusses heterolytic splitting of H-H, Si-H, and other σ bonds on electrophilic metal centers. Extensive computational analyses of the structure, bonding, and reactions of coordinated dihydrogen are carried out because of the innate ‘‘simplicity’’ of the H2 ligand. However, this is quite deceptive because M-H2 systems have proven to exhibit astonishingly complex structure, bonding, and dynamics, including quantum mechanical behavior. In principle, any X-Y σ bond can coordinate to a metal center, providing that steric and electronic factors are favorable—for example, substituents at X and Y do not block the metals access. Heterolytic splitting of X-H bonds via proton transfer to a basic site on a cis ligand or to an external base is a crucial step in both industrial and biological processes. Calculations show that for highly electrophilic M the reduction in back donation is almost completely offset by increased electron donation from H2 to the electron-poor M. One of the old, most significant, and widespread reactions of H2 on metal centers is heterolytic cleavage, which involves essentially breaking the H-H bond into H+ and H– fragments.

Catalytic Processes Involving Dihydrogen Complexes and Other Sigma-bond Complexes

The discovery of dihydrogen complexes, LnM(H2), pointed to direct transfer of hydrogen from coordinated H2 ligands to substrates as an operable pathway in catalysis both in homogeneous and heterogeneous systems. Sigma complexes, LnM(η2-H–X) (X=H, Si, C, etc), are indeed relevant in hydrogenation as well as silane alcoholysis and methane conversion.

H2 binding to and silane alcoholysis on an electrophilic Mn(I) fragment with tied-back phosphite ligands. X-ray structure of a Mn–CH2Cl2 complex

Abstract The solvent-coordinated cationic complex [mer-Mn(CO)3{P(OCH2)3CMe}2(ClCH2Cl)][BArF] (4), has been prepared by the reaction of the methyl precursor mer-Mn(Me)(CO)3{P(OCH2)3CMe}2 with [Ph3C][BArF]. The coordinated solvent, CH2Cl2, binds to Mn through one chloride atom in the X-ray crystal structure, which also exhibits novel interligand hydrogen bonding between an acidic hydrogen on CH2Cl2 and an oxygen of the phosphite. 4 binds H2 in equilibrium fashion, and the η2-H2 complex has a very high JHD of 34.5 Hz indicative of the high electrophilicity of the metal center. Silanes also displace the bound CH2Cl2 at low temperature, although the η2-SiH bond undergoes heterolytic cleavage on warming. 4 catalyzes reaction of SiHEt3 with phenol to give Et3SiOPh and H2. The bound CH2Cl2 in 4 is displaced irreversibly by olefins, ethers, and amines, to form stable adducts. The cationic [Mn(CO)3(P(OCH2)3CMe)2]+ fragment is more electrophilic than phosphine analogues, and the tied-back phosphites give less steric congestion and, importantly, cannot engage in agostic interactions that would compete with external ligand binding. The results in these and other related systems bring to the forefront the subtle balance between electronic and steric forces that occur on addition of sixth ligands to 16 e− metal fragments.

Molecular Hydrogen Coordination to Transition Metals

Abstract The discovery of molecular hydrogen coordination to transition metals and its significance in terms of reactions of σ bonds at metal centers and catalysis is described. The fact that several complexes, known for many years, have only now been shown to contain H2 ligands after our finding, perhaps best reflects how surprising and well hidden this phenomenon has been. The existence of a tautomeric-like relationship between dihydrogen and dihydride ligands was equally unexpected. Diagnostics for, properties of. and induced cleavage of H2 ligands are given, along with a bonding model in harmony with these properties.

Characterization of transition-metal molecular hydrogen complexes by solid-state proton NMR

Recently it has been discovered that stable transition-metal complexes containing molecular hydrogen as a ligand can be prepared. Current research under way in several laboratories indicates that the eta/sup 2/ mode of binding hydrogen is fairly common and even occurs in several polyhydrides previously believed to be classical in structure. These molecules have been cited as examples of an arrested oxidative addition of hydrogen to a metal complex, and the observed variations in physical properties seems to indicate that the addition is halted at different points. Characterization of these eta/sup 2/-dihydrogen complexes has relied on solution NMR or single-crystal diffraction studies to verify that the H/sub 2/ ligand remains intact when bound. Interpretation of solution NMR results is complicated by the fluxional nature of these species, and location of the hydrogens by diffraction is hampered by anisotropic motion and disorder of the H/sub 2/ ligand. In this paper they report on a simple, generally applicable solid-state NMR method that is well suited for characterizing these species. The method uses the large dipolar couplings between the H/sub 2/ protons to determine their separation and does not require single crystals or deuteration of the other coordinating ligands.

Rotational tunneling of bound H2 in a tungsten complex

The rotational tunnel splitting of the librational ground state of activated molecular hydrogen bound in a tungsten complex has been determined to be 0.95 (1) cm−1. With the assumption of a twofold cosine potential with one degree of freedom for the rotation, this result yields a barrier height of 760 cm−1 in good agreement with the analysis of the librational data. These quantum mechanical rotations are also found to persist up to temperatures above 150 K. The relationship of these measurements to the binding of molecular hydrogen to the metal center are discussed.

Metal complexes based on an upper-rim calix[4]arene phosphine ligand

Abstract A new upper-rim phosphacalix[4]arene 5,17-bis(diphenylphosphinomethyl)-25,26,27,28-tetrapropoxycalix[4]arene ( 4 ) has been prepared starting from commercially available tert -butyl calix[4]arene. Treatment of 4 with (COD)PdMeCl and (COD)PtCl 2 gives polymeric phosphine-coordinated Pd(II) and Pt(II) species, respectively. 4 reacts with [(COD)RhCl] 2 to give a di-rhodium complex that is an active catalyst for the hydroformylation of 1-octene and styrene.

Vibrational analysis using crystalline models: Vibrational spectra and potential constants for Cs2LiFe(CN)6

Single crystal Raman and infrared powder spectra have been observed for Cs2LiFe(CN)6, and for species Cs2LiFe(13CN)6 and Cs2LiFe(C15N)6 enriched to 94% 13C and 99% 15N, respectively. The infrared spectrum of the 6Li isotopically enriched species has also been observed. The above vibrational data have been used in a normal mode calculation including interionic interaction potential constants and lattice modes. While the overdetermined A1g and Eg symmetry blocks converge readily, the 6×6 F1u symmetry block is ill defined. A comparison of the potential constants obtained for Cs2LiFe(CN)6 with those obtained earlier for Co(CN)6−3 shows that the M–C sigma bonding is slightly weaker for the iron complex.