Solid State and Solution NMR of Nonclassical Transition-Metal Polyhydrides

Unravelling strong temperature-dependence of JHD in transition metal hydrides: solvation and non-covalent interactions versus temperature-elastic H-H bonds

A number of transition metal hydrides reveal intriguing temperature-dependent JHD in their deuterated derivatives and possibly the temperature dependent hydrogen-hydrogen distance (r(H-H)) as well. Previously, theoretical studies rationalized JHD and r(H-H) changes in such compounds through a "temperature-elastic" structure model with a significant population of vibrational states in an anharmonic potential. Based on the first variable temperature neutron diffraction study of a relevant complex, (p-H-POCOP)IrH2, observation of its elusive counterpart with longer r(H-H), crystallized as an adduct with C6F5I, and thorough spectroscopic and computational study, we argue that the model involving isomeric species in solution at least in some cases is more relevant. The existence of such isomers is enabled or enhanced by solvation and weak non-covalent interactions with solvent, such as halogen or dihydrogen bonds. "Non-classical" hydrides with r(H-H) ≈ 1.0-1.6 Å are especially sensitive to the above-mentioned factors.

Experimental characterization of the hydride 1H shielding tensors for HIrX2(PR3)2 and HRhCl2(PR3)2: extremely shielded hydride protons with unusually large magnetic shielding anisotropies

The hydride proton magnetic shielding tensors for a series of iridium(III) and rhodium(III) complexes are determined. Although it has long been known that hydridic protons for transition-metal hydrides are often extremely shielded, this is the first experimental determination of the shielding tensors for such complexes. Isolating the (1)H NMR signal for a hydride proton requires careful experimental strategies because the spectra are generally dominated by ligand (1)H signals. We show that this can be accomplished for complexes containing as many as 66 ligand protons by substituting the latter with deuterium and by using hyperbolic secant pulses to selectively irradiate the hydride proton signal. We also demonstrate that the quality of the results is improved by performing experiments at the highest practical magnetic field (21.14 T for the work presented here). The hydride protons for iridium hydride complexes HIrX2(PR3)2 (X = Cl, Br, or I; R = isopropyl, cyclohexyl) are highly shielded with isotropic chemical shifts of approximately -50 ppm and are also highly anisotropic, with spans (=δ11 - δ33) ranging from 85.1 to 110.7 ppm. The hydridic protons for related rhodium complexes HRhCl2(PR3)2 also have unusual magnetic shielding properties with chemical shifts and spans of approximately -32 and 85 ppm, respectively. Relativistic density functional theory computations were performed to determine the orientation of the principal components of the hydride proton shielding tensors and to provide insights into the origin of these highly anisotropic shielding tensors. The results of our computations agree well with experiment, and our conclusions concerning the importance of relativistic effects support those recently reported by Kaupp and co-workers.

Dihydrogen complexes as prototypes for the coordination chemistry of saturated molecules

The binding of a dihydrogen molecule (H(2)) 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 H H and C H. Before the seminal finding of side-on bonded H(2) in W(CO)(3)(PR(3))(2)(H(2)), it was generally believed that H(2) 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 H(2), silanes, and alkanes (sigma-complexes) have a chemistry of their own, with surprisingly varied structures, bonding, and dynamics. H(2) complexes are of increased relevance for H(2) production and storage in the hydrogen economy of the future.

Effect of weak interactions on the H...H distance in stretched dihydrogen complexes

Computational and experimental results presented in this paper demonstrate that the H-H distance in stretched dihydrogen complexes can be hypersensitive to a variety of weak intra- and intermolecular interactions, including those with bulky ligands and solvent molecules, hydrogen-bonding interactions, or ion-pairing. Particularly, the complex IrH(H...H)Cl(2)(P(i)Pr(3))(2) which contains a stretched dihydrogen ligand in the crystalline form, as shown by neutron diffraction, is a trihydride in solution. The difference is due to the intermolecular Ir-Cl...H-Ir hydrogen bonding in the solid.

Elongated dihydrogen complexes: what remains of the H-H bond?

Of the several hundred examples of transition metal dihydrogen complexes that have been reported to date, the vast majority have H-H distances of less than 1.0 Angstrom. A small number of complexes have been reported with distances in the range of 1.1 to 1.5 Angstrom. These complexes have been termed elongated dihydrogen complexes. In this review, experimental methods for structure determination of such complexes are summarized, along with computational approaches which have proven useful in understanding the structures of these molecules.

Intramolecular rearrangements in six-coordinate ruthenium and iron dihydrides

Molecular orbital calculations at the ab initio level are used to study polytopal rearrangements in H2Ru(PH3)4 and H2Fe(CO)4 as models of 18-electron, octahedral metal dihydrides. It is found that, in both cases, the transition state for these rearrangements is a dihydrogen species. For H2Fe(CO)4, this is a square pyramidal complex where the H2 ligand occupies an apical position and is rotated by 45 degrees from its original orientation. This is precisely analogous to the transition state for Fe-olefin rotation in (olefin)Fe(CO)4 complexes and has a very similar electronic origin. Another transition state very close in energy is found wherein the basic coordination geometry is a trigonal bipyramid and the H2 ligand is coordinated in the axial position. For H2Ru(PH3)4, the former stationary point lies at a much higher energy and the latter clearly serves as the transition state for hydride exchange. The reason for this difference is discussed along with the roles of electron correlation in the two compounds.

Reversible Displacement of Polyagostic Interactions in 16e [Mn(CO)(R(2)PC(2)H(4)PR(2))(2)](+) by H(2), N(2), and SO(2). Binding and Activation of eta(2)-H(2) trans to CO Is Nearly Invariant to Changes in Charge and cis Ligands

Electrophilic 16e [Mn(CO)(R(2)PC(2)H(4)PR(2))(2)](+) complexes (R = Et, Ph) are synthesized by metathesis of MnBr(CO)(R(2)PC(2)H(4)PR(2))(2) with Na or Li salts of low-coordinating boron or gallium anions (e.g., [B{C(6)H(3)(3,5-CF(3))(2)}(4)](-) or [Ga(C(6)F(5))(4)] (-)). They contain weak polyagostic interactions that are reversibly displaced by H(2), N(2), and SO(2) (which is a surprisingly weak ligand here). The agostic and H(2) complexes, as well as the gallium anions including the new species [{Ga(C(6)F(5))(3)}(2)(&mgr;-Cl)](-), have been characterized by NMR, IR, and X-ray crystallography. The agostic Mn-H distances (e.g., 2.9 Å) are much longer than those found for the single agostic interactions in Mo(CO)(diphosphine)(2) and [Mn(CO)(3)(PCy(3))(2)](+). The H-H and also the Mn-H distances have been determined in the H(2) complex by T(1) measurements for both the H(2) and HD isotopomers. IR data and C-O and M-C bond lengths are used to gauge the pi-acceptor strengths of ligands trans to the CO. The agostic C-H bonds are the weakest ligands and also the weakest acceptors, but the H(2) ligand is an excellent acceptor as strong as N(2) and ethylene. The variation of nu(CO) on increasing the basicity of the cis-phosphine (dppe versus depe) in trans-M(CO)(diphosphine)(2)(L) is less than expected and far less than that on increasing the charge on the complex (M = Mn(+) versus Mo). The H-H bond lengths (0.87-0.90 Å) and J(HD) NMR couplings (32-34 Hz) in [Mn(CO)(R(2)PC(2)H(4)PR(2))(2)(H(2))](+) and other cationic H(2) complexes with trans-CO are strikingly similar to their neutral analogues and nearly invariant. Activation of H-H in the more electrophilic cationic systems occurs primarily via increased sigma donation from H(2) as compared to the more electron-rich neutral analogues where back-bonding dominates. The nature of the ligand trans to H(2) (the strong acceptor CO here) controls the H-H distance more so than all of the cis ligands combined, especially for cationic complexes.

Comparison of H-H versus Si-H sigma-Bond Coordination and Activation on 16e Metal Fragments. Organosilane, N(2), and Ethylene Addition to the Agostic Complex W(CO)(3)(PR(3))(2) and Dynamic NMR Behavior of the Latter

Variable-temperature (31)P{(1)H} NMR spectroscopy of the agostic complexes M(CO)(3)(PCy(3))(2) (M = Mo, W) indicates dynamic behavior as evidenced by collapse below -20 degrees C of a singlet to an AB signal plus a shifted singlet. The inequivalency of the phosphines is possibly due to the presence of conformational isomers resulting from hindered rotation of the M-P bond or, less likely, a geometric isomer with pseudo-cis PCy(3) ligands. Further studies on the coordination chemistry of W(CO)(3)(PR(3))(2) (R = iPr, Cy) were performed. The bridging dinitrogen complex [W(CO)(3)(PiPr(3))(2)](2)(&mgr;-N(2)) (1) was cleanly formed in the reaction of W(CO)(3)(PiPr(3))(2) with N(2). Complex 1 was structurally characterized and compared with other bridging dinitrogen compounds of tungsten. The ethylene complex W(CO)(3)(PCy(3))(2)(eta(2)-C(2)H(4)) (2) was synthesized and characterized by X-ray crystallography in order to compare the binding mode of ethylene with that of H(2). Phenylsilane reacted with W(CO)(3)(PR(3))(2) (R = iPr, Cy) to form the thermally unstable oxidative addition (OA) products WH(SiH(2)Ph)(CO)(3)(PR(3))(2) (3, R = Cy; 4, R = iPr). Diphenylsilane reacted with W(CO)(3)(PiPr(3))(2) at 60 degrees C to form the bridging silyl species [W(CO)(3)(PiPr(3))(&mgr;-SiHPh(2))](2) (5), which was confirmed by spectroscopic techniques and X-ray crystallography to have two 3-center 2-electron W.H.Si interactions. Detailed comparisons of the binding and activation of silanes versus H(2) on various 16e metal centers suggest a high degree of similarity, but relative ease of OA depends on the electrophilicity of the metal-ligand fragment and other factors such as bond energetics. Increasing the electrophilicity of the metal center (e.g., adding positive charge) may aid in stabilizing alkane coordination.

Structure and H(2)-Loss Energies of OsHX(H(2))(CO)L(2) Complexes (L = P(t-Bu)(2)Me, P(i-Pr)(3); X = Cl, I, H): Attempted Correlation of (1)J(H-D), T(1min), and DeltaG()

1J(H-D), T(1min) and k(1) for H(2) dissociation from OsHX(H(2))(CO)L(2) have been measured for X = Cl, I, H (L = P(t-Bu)(2)Me or P(i-Pr)(3)), as well as for OsCl(2)(H(2))(CO)(P(i-Pr)(3))(2). For comparison, new data (including previously unobserved coupling constants) have been reported for W(HD)(CO)(3)(P(i-Pr)(3))(2). A comprehensive consideration of T(1min) data for over 20 dihydrogen complexes containing only 1-2 phosphines cis to H(2), together with a consideration of the shortest "conceivable" H-H distance for H(2) bound to a d(4) or d(6) metal, is used to argue that the "fast spinning" model is not appropriate for determining r(H-H) in such complexes. Regarding OsHX(H(2))(CO)L(2), the stronger electron-donor (lighter) halide, when cis to H(2), facilitates loss of H(2). The complete absence of pi-donor ability when X = H renders H(2) loss most difficult. However, a pi-donor trans to H(2) also makes H(2) loss unobservable. Within the series of isoelectronic, structurally analogous Os complexes, a longer H-H bond shows a larger DeltaG() for H(2) loss. However, this correlation does not continue to W(H(2))(CO)(3)(P(i-Pr)(3))(2), which has r(H-H) comparable to that of OsH(halide)(H(2))(CO)(P(i-Pr)(3))(2), but a significantly higher DeltaG(). This may originate from lack of a pi-donor ligand to compensate as H(2) leaves W.

1995 Alcan Award Lecture New intermediates in the homolytic and heterolytic splitting of dihydrogen

Some of the research of the author and his research group into the structure and reactions of dihydrogen complexes of transition metals is reviewed. The characterization of osmium complexes that can be regarded as having intermediate structures on the way to the homolytic splitting and to the heterolytic splitting of dihydrogen is described. The properties of an iridium complex with novel short proton–hydride contacts is also reviewed. Key words: transition metal, dihydrogen, hydride, complexes, NMR, neutron diffraction, osmium, iridium.