Transition metal hydride

Transition metal hydrides are chemical compounds containing a transition metal bonded to hydrogen. Most transition metals form hydride complexes and some are significant in various catalytic and synthetic reactions. The term "hydride" is used loosely, some so-called hydrides are acidic and some are hydridic.

The first metal hydrides to be characterized were H₂Fe(CO)₄ and HCo(CO)₄, in work by Walter Hieber in the 1930's. After a hiatus of several years, several new hydrides were reported in the mid-1950's by three prominent groups in organometallic chemistry: HRe(C₅H₅)₂ by Wilkinson, HMo(C₅H₅)(CO)₃ by Fischer, and HPtCl(PEt₃)₂ by Chatt. [H. D. Kaesz, R. B. Saillant “Hydride Complexes of the Transition Metals” Chemical Reviews, 1972, Vol. 72, 231-281.] Thousands of such compounds are now known.

ynthesis

Hydride transfer

"Powerful" main group hydrides deliver hydrides to give less hydridic transition metal complexes in metathesis reactions::ML_nX + MHBEt₃ → HML_n + BEt₃ + MX

Elimination reactions

beta-hydride elimination and alpha-hydride elimination The former a common termination pathway in homogeneous polymerization. It also allows some transition metal hydride complexes to be synthesized from organolithium and Grignard reagents.:ML_nX + LiC₄H₉ → C₄H₉ML_n + LiX:C₄H₉ML_n → HML_n + H₂C=CHC₂H₅

Oxidative additions

Addition of dihydrogen to a transition metal center is perhaps the most classic example of oxidative addition::ML_n + H₂ ⇌ H₂ML_nOxidative addition also can occur to dimetallic complexes:: [ML_n] ₂ + H₂ ⇌ 2 HML_n

Protonation of a complex is viewed as an oxidative process as well:: ML_n + H⁺ ⇌ HML_n⁺

Heterolytic cleavage of dihydrogen

A hydride can be formed by splitting a hydrogen molecule between a hydride and proton acceptor. The mechanism for this reaction most likely involves the addition of hydrogen to the metal complex to form a classical or nonclassical dihydride followed by deprotonation. This is also an important mechanism for bifunctional catalysts, some of which are believed to react through a concerted mechanism.

:ML_n^x+ + Base + H₂ ⇌ HML_n^(x-1)+ + HBase⁺

Thermodynamic considerations

The M-H bond can, in principle, cleave to produce a proton, hydrogen radical, or hydride. [DuBois, D.L., Blake, D.M., Miedaner, A., Curtis, C.J., Rakowski DuBois, M., Franz, J.A., and Linehan, J.C. "Organometallics", 2006, "25", 4414-4419.] [Curtis, C. J.; Miedaner, A.; Raebiger, J. W.; DuBois, D. L. "Orgnometallics", 2004, "23", 511-516.] [Curtis, C. J.; Miedaner, A.; Ciancanelli, R. F.; Ellis, W. W.; Noll, B. C.; Rakowski DuBois, M.; DuBois, D. L. "Inorg. Chem." 2003, "42", 216-227.] [Curtis, C. J.; Miedaner, A.; Ellis, W. W.; DuBois, D. L. "J. Am. Chem. Soc." 2002, "124", 1918-1925.]

:HML_n ⇌ ML_n^- + H⁺

:HML_n ⇌ ML_n + H

:HML_n ⇌ ML_n⁺ + H^-

Although these properties are interrelated, they are not interdependent. A metal hydride can be a thermodynamically a weak acid and a weak H^- donor; it could also be strong in one category but not the other or strong in both. The H^- strength of a hydride also known as its hydride donor ability or hydricity corresponds to the hydride's Lewis base strength. Not all hydrides are powerful Lewis bases. The base strength of hydrides vary as much a the pKa of protons. This hydricity can be measured by heterolytic cleaving hydrogen between a metal complex and base with a known pK_a then measuring the resulting equilibrium. This presupposes that the hydride doesn't heterolytically or homolytically react with itself to reform hydrogen. A complex would homolytically react with itself if the homolytic M-H bond is worth less than half of the homolytic H-H bond. Even if the homolytic bond strength above that threshold the complex is still susceptible to radical reaction pathways.

:2 HML_n^z ⇌ 2 ML_n^z + H₂

A complex will heterolytically react with itself when its simultaneously a strong acid and a strong hydride. This conversion results in disproportionation producing a pair of complexes with oxidation states that differ by two electrons. Further electrochemical reactions are possible.

:2HML_n^z ⇌ ML_n^z+1 + ML_n^z-1 + H₂

As noted some complexes heterolytically cleave dihydrogen in the presence of a base. A portion of these complexes result in hydride complexes acidic enough to be deprotonated a second time by the base. In this situation the starting complex can be reduced by two electrons with hydrogen and base. Even if the hydride is not acidic enough to be deprotonated it can homolytically react with itself as discussed above for an overall one electron reduction.

:Two deprotonations: ML_n^z + H₂ + 2Base ⇌ ML_n^z-2 + 2H⁺Base

:Deprotonation followed by homolysis: 2ML_n^z + H₂ + 2Base ⇌ 2ML_n^z-1 + 2H⁺Base

The H^- bond strength (the hydricity) of M-H fragments have been demonstrated to be greatest for first row transition metal centers and least for second row transition metal centers with third row metal centers falling in between for corresponding complexes. These results run contrary to tenants of ligand field theory which suggest that metal ligand bond strengths generally increase as a periodic group is descended.

Kinetics and mechanism

The rates of proton-transfer to and between metal complexes are often slow. [Kramarz, K. W. and Norton, J. R., "Slow Proton Transfer Reactions in Organometallic and Bioinorganic Chemistry", Prog. Inorg. Chem., 1994, 42, 1-65] Many hydrides are inaccessible to study through Bordwell thermodynamic cycles. As a result, kinetic studies are employed to elucidate both the relevant thermodynamic parameters. Generally hydrides derived from first row transition metals display the most rapid kinetics followed by the second and third row metal complexes.

Bonding motifs

Metal complexes containing terminal hydrides are common. Complexes featuring bridging are also known. Of these bridging hydrides many are oligomaric, such as Stryker's reagent [Chiu, P.;Li, Z.; Fung, K. C. M. "Tetrahedron Letters" 2003, "44", 455-457.] [(Ph₃P)CuH] ₆ and clusters such as [Rh₆(PR₃)₆H₁₂] ²⁺ [Brayshaw, S.; Harrison, A.; McIndoe, J.; Marken, F.; Raithby, P.; Warren, J.; Weller, A. "J Am Chem Soc." 2007, "129", 1793-1804.] . The final bonding motif is the non-classical dihydride also known as sigma bond dihydrogen adducts or simple a dihydrogen complex. The [W(PR₃)₂(CO)₃(H₂)] complex was the first well characterized example of both a non-classical dihydride and sigma-bond complex in general. [Kubas, G. J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.; Wasserman, H. J. "J. Am. Chem. Soc." 1984, "106", 451-452.] [Kubas, G. J. Metal Dihydrogen and sigma-Bond Complexes: Structure, Theory and Reactivity; Kluwer Academic/Plenum Publishers: New York, 2001.] X-ray diffraction is generally insufficient to locate hydrides in crystal structures and thus their location must be assumed. It requires Neutron diffraction to unambigously cytalographically locate a hydride near a heavy atom. Non-classical hydrides have also been studied with a variety of variable temperature NMR techniques and HD Couplings.

:Classical Terminal: M-H

:Classical Bridging: M-H-M

:Non-Classical: M-H₂

Characteristic properties

Late transition metal hydrides are known for their down-field shifts in the proton NMR spectrum. It is common for the M-H signal to appear between δ-5 and -25 with many examples outside this range but generally all appear below 0 ppm. ¹H NMR signals for early transition metal hydrides can appear from upfield to far downfield. Since hydrides can act as powerful bases, many react with solvents which can supply a proton. This includes protic solvents such as water and alcohols but also chlorinated solvents such as chloroform and methylene chloride. Since these chlorinated solvent usually produce protons as HCl the metal centers hydride is often exchanged for a chloride during the production of hydrogen.

References