PCA ETR:PERSPECTIVE FEATURE: SPECIAL

Dihydrogen complexes as prototypes for the coordination of saturated molecules

Gregory J. Kubas† Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM 87545

Edited by Jay A. Labinger, California Institute of Technology, Pasadena, CA, and accepted by the Editorial Board February 26, 2007 (received for review November 14, 2006)

The binding of a dihydrogen molecule (H2) to a 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. Be- fore 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 . Metal- bound saturated molecules such as H2, silanes, and (␴-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.

ihydrogen (H2) and hydrocar- bons are vital in chemical processes such as hydrogena- tion and conversions of organicD compounds. Catalytic hydro- genations are the largest-volume chemi- cal reactions: all crude oil is treated Sketch 1. Eq. 1. with H2 to remove sulfur/nitrogen, and Ͼ100 million tons of ammonia fertilizer buckminsterfullerene (C ) (6). That its are produced annually to support much 60 existence remained hidden for so long cyclohexyl) (9). Its unusual purple color of the world’s population. The H mole- 2 adds to the lore, and our unexpected changed instantly and reversibly to yel- cule is married together by a very strong revelation of metal–H2 complexes has low on exposure to N2 and H2 in both two- H–H bond but is only use- some commonality. Metal dihydrides ful chemically when the two H atoms solution and solid states, signifying ad- formed by (OA) of duct formation (Eq. 1). divorce in controlled fashion. This also the H–H bond to a metal center had applies to other strong ␴-bonds such as later revealed a phos- been known early on to be a part of phine C–H bond weakly occupying the C–H in alkanes. However, the mecha- catalytic cycles (7), as documented in sixth binding site in W(CO)3(PCy3)2 nism at the molecular level by which the a 1980 retrospective on catalytic hydro- union splits was established only rela- (10). This type of ‘‘agostic’’ interaction genation by a pioneer in the field, Jack (11) relieves electronic unsaturation in tively recently because such electroni- Halpern (8). Although some form of coordinatively unsaturated complexes cally saturated molecules were never metal–H interaction was assumed to 2 and is entropically favorable because it caught in the act of chemically binding participate in dihydride formation, it to a metal or other ‘‘third party,’’ usu- was thought to be unobservable. We is ‘‘intramolecular.’’ ‘‘Intermolecular’’ ally the first step in breaking apart a were fortunate to observe it in the com- binding of a C–H bond as in an strong bond. The discovery by Kubas ␴-complex (often also termed ‘‘agostic’’) plex W(CO)3(PR3)2(H2), as detailed by and coworkers (1) in 1984 of coordina- this author (2, 3). This was the first mo- is less stable. Irrefutable evidence for tion of a nearly intact H2 molecule to a lecular compound synthesized and iso- H2 binding in Eq. 1 came slowly because ϭ metal complex (LnM; L ) lated entirely under ambient conditions pinpointing H positions crystallographi- caught this in intimate detail and led to that contained the H2 molecule (albeit cally is difficult, even by neutron diffrac- a new paradigm in chemistry (1–4) (see ‘‘stretched’’) other than elemental H2 tion. A consultant, Russ Drago, sug- Sketch 1). itself. The H–H bond length in gested an experiment elegant in its ␩2 The H2 binds side-on ( ) to M pri- W(CO) (PiPr ) (H ) (0.89 Å) is simplicity: synthesize the HD complex ␴ 3 3 2 2 marily through donation of its two stretched Ϸ20% over that in H2 (0.74 and look for a large HD coupling con- to a vacant metal orbital to Å), showing that H2 is not physisorbed stant in the proton NMR that would form a stable H2 complex. It is remark- but rather chemisorbed, where the bond show that the H–D bond was mostly able that the electrons already strongly is ‘‘activated’’ toward rupture. Like H2, intact. It worked beautifully: the 1H bonded can donate to a metal to form a i other saturated molecules such as al- NMR of W(CO)3(P Pr3)2(HD) showed a nonclassical 2-electron, 3-center bond, kanes were thought to be inert to such 1:1:1 triplet (deuterium spin ϭ 1) with as in other ‘‘electron-deficient’’ mole- binding, although their C–H bonds JHD ϭ 33.5 Hz, nearly as high as in HD cules such as diborane (B2H6). M–H2 somehow could also be broken on met- ␴ gas, 43.2 Hz. Observation of JHD higher and other ‘‘ -complexes’’ (3, 5), encom- als. The ‘‘somehow’’ is why the finding Ͼ ␴ than that for a dihydride complex ( 2 passing interaction of any -bond (C–H, of an H2 complex was important: it is Si–H, etc) with a metal center, are the the prototype for activation of all major theme of this special feature. ␴-bonds. Author contributions: G.J.K. wrote the paper. i This discovery of W(CO)3(P Pr3)2(H2) The author declares no conflict of interest. Introduction and Historical Perspective ensued the serendipitous synthesis of its This article is a PNAS Direct Submission. J.A.L. is a guest Certain discoveries and how they came novel, ‘‘unsaturated’’ 16-e precursor, editor invited by the Editorial Board. about are fascinating sagas, e.g., that for M(CO)3(PCy3)2 (M ϭ Mo, W; Cy ϭ †E-mail: [email protected].

www.pnas.org͞cgi͞doi͞10.1073͞pnas.0609707104 PNAS ͉ April 24, 2007 ͉ vol. 104 ͉ no. 17 ͉ 6901–6907 Downloaded by guest on September 25, 2021 Eq. 2. Scheme 1.

Hz) became the premier criterion for an and Heinekey. This quartet has since H2 complex. performed elegant synthetic, reactivity, more characteristic of a dihydride, which One reason that H2 complexes were and NMR studies on H2 and silane it was initially believed to be (47). Com- so well hidden was the notion that such complexes (5, 25–30) and was eventually plexes containing only H2O (48) or CO complexes could not be stable relative joined by Ͼ100 investigators worldwide. (17, 18) coligands are also known, but to classical dihydrides, as exemplified by Remarkably, several complexes initially are marginally stable (Scheme 1). the controversy over our initial findings. believed to be were revealed to Determining the presence of a H2 ligand and its d is nontrivial because This paralleled the discovery of metal– be H2 complexes by Crabtree and Ham- HH dinitrogen complexes by Allen and Se- ilton in 1986 (5, 31), by using as criteria even neutron diffraction has limited ap- noff, whose seminal paper was initially the short proton NMR relaxation times plicability and can give foreshortened rejected (12). At the time of our finding, of H (T Ͻ 100 msec). Particu- dHH because of rapid H2 rotation/libra- 2 1 1 tion (49). JHD is the best criterion, and spectroscopic evidence for unstable larly interesting was RuH2(H2)(PPh3)3 values determined in solution correlate M–H2 interactions was found by photol- first reported in 1968 (32); it possessed well with dHH in the solid state through ysis of Cr(CO)6 in the presence of H2 at unusual H2 lability that Singleton in Eqs. 3 and 4 (50, 51). low T (13–16). Cr(CO)5(H2) was postu- 1976 commented was characteristic of lated based on IR CO stretching fre- ‘‘H2-like bonding’’ (33). However, at- ϭ Ϫ ͓ ͔ dHH 1.42 0.0167JHD Å Morris quencies but could not be conclusively tempts to prove H2 binding here was 1 demonstrated; only recently has its H problematic, even long after H2 binding [3] NMR spectrum been observed at low T was established (34).‡ ϭ (17, 18). Even theoretical bases for in- More than 600 H complexes are dHH 1.44 ␴ 2 teraction of H2 and other -bonds with known (most of them stable) for nearly Ϫ 0.0168J Å ͓Heinekey͔. a metal was still in its infancy at the every transition metal and type of coli- HD time of our discovery. Ironically, a com- gand and are the focus of 1,500 publica- [4] putational paper by Saillard and Hoff- tions, dozens of reviews, and three mann (19) in 1984 on the bonding of H2 monographs (2–5, 20, 25–30, 35–43). Data include dHH from crystallography and CH4 to metal fragments such as The view on H2 complexes has shifted and also solid-state NMR measurements Cr(CO)5 was published shortly after our from significance in basic science to a by Zilm and Millar (52) that gave the publication (1) of the W–H2 complex, more practical bent, e.g., H2 fuel pro- most accurate dHH (direct measure of without mutual knowledge. Such inter- duction and storage. Two frequent internuclear HH separation). For i ϭ play between theory and experiment has questions after their discovery were as W(CO)3(P Pr3)2(H2), JHD 34 Hz, giv- ϭ continued as one of the most valuable follows. Are H2 complexes relevant in ing dHH 0.86–0.88 Å vs. 0.89 Å from synergistic relations in all of chemistry catalysis, i.e., does direct transfer of hy- solid-state NMR and 0.82(1) Å from (3, 4, 20). The innate simplicity of H 2 drogen from an H2 ligand to a substrate neutron diffraction [uncorrected for H2 was attractive computationally, but the occur? And could methane bind to libration (49)]. Short T1 values for the structure/bonding/dynamics of H2 com- metal complexes? The answer to both H2 ligand (27) are also diagnostic (e.g., plexes turned out to be unimaginably is yes, and although a stable methane 4 msec for the W complex), although complex and led to extensive study complex has not been isolated, alkane care must be exercised in interpretation Ͼ ( 300 computational publications). binding has been observed. (53–55). A powerful spectroscopic tool Initially, H2 binding in M(CO)3 developed by a colleague at Los (PR3)2(H2) seemed unique because the Synthesis and Diagnosis of H2 Complexes Alamos, Juergen Eckert, is inelastic bulky phosphines sterically inhibited for- neutron scattering studies of rapid H Most H2 complexes contain low-valent 2 mation of a 7-coordinate dihydride metals with d6 electronic configurations rotation in solid H2 complexes that pro- through OA. Kaesz and coworkers (21) that favor side-on binding of ␴-bonds. vide unequivocal evidence for molecular viewed this as ‘‘arrested OA,’’ a descrip- Reversibility of H binding is often a H2 binding and also the presence of 2 Ͼ tive term for the bonding in a silane key feature, i.e., H can be removed M– H2 backdonation (56). ␩2 2 complex, CpMn(CO)2( -HSiPh3). Si- simply upon exposure to vacuum and Several synthetic routes to H2 com- lane complexes (22, 23) were among the plexes are available; the simplest is reac- ␴ readded many times at ambient temper- first examples of -bond complexes but ature/pressure, as in Eq. 1. Virtually all tion of H2 with an unsaturated complex were initially unrecognized as such be- ␴-complexes are diamagnetic, with one such as W(CO)3(PR3)2 (Eq. 1). Displace- cause the asymmetrically bound silane exception (44). ␴-Ligands have not been ment of weakly bound ‘‘solvento’’ ligands ligand (Eq. 2) lacked the superb clarity such as CH2Cl2 (57) or H2Ofrom definitively shown to bridge metals. Sur- 2ϩ of the H2 ligand, which has electrons [Ru(H2O)6] (Scheme 1) is also effective. prisingly, the coligands on H2 complexes only in the H–H bond. can be simple classical nitrogen-donor Protonation of a complex by acids The hundreds of H2 complexes syn- ancillary ligands such as ammonia, as in is most often used (28, 38) and is widely thesized after our discovery could not [Os(NH ) (H )]2ϩ (45), which has a very applicable because it does not require an initially have been imagined, and it was 3 5 2 unsaturated precursor that may not be long H–H distance (dHH), Ϸ1.34 Å (46), difficult to know where to search for available. Neutral polyhydrides LnMHx are new ones. It would take more than convenient targets for protonation to cat- ϩ ‡ 1 a year before others were identified, In 1993 Zilm obtained solid-state H NMR evidence for H2 ionic H2 complexes, [LnM(H2)Hx-1] , notably by Morris, Crabtree, Chaudret, coordination (dHHϭ 0.93 Å) on a sample we prepared. which can be more robust than complexes

6902 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0609707104 Kubas Downloaded by guest on September 25, 2021 O O P P C C Cl H HCl Cl P CO CO Ir Ir H W P W H H H Cl P P P H P H H H C C H–H = < 0.9 Å H–H = 1.11Å O O Scheme 4. Eq. 5.

Scheme 2. with T for the former and decreases for M, the ␴-bond cleaves to form a dihy- the latter (69). These are terms because dride because of overpopulation of the a near continuum of dHH has been ␴ prepared from H2. Only a few stable H2 * orbital. There is often a fine line observed. solid bis-H2 complexes are known, e.g., between H2 and dihydride coordination, Activation of H2 is very sensitive to ϩ [RhH2(H2)2(PCy3)2] (58), Tp*RuH(H2)2 and in some cases equilibria exist in so- M, L, and charge, e.g., changing R from (59), and RuH2(H2)2(PR3)2;Rϭ Cy (30) lution for W(CO)3(PR3)2(H2) (Eq. 5), phenyl to alkyl in Mo(CO)(H2) and cyclopentyl (60), for which the neu- showing that side-on coordination of H2 (R2PC2H4PR2)2 leads to splitting of H2 tron structure shows unstretched cis–H2 is the first step in H–H cleavage (2). (49). Strongly donating L, third-row M, ligands. Although electronic factors for OA and neutral charge favor elongation or are well established, the role of steric splitting of H–H, whereas first-row M, Structure, Bonding, and Dynamics of H2 factors is less clear. Bulky phosphines electron-withdrawing L, and positive Complexes can inhibit H2 splitting: for less bulky charge (cationic complex) favor H2 ϭ The 3-center metal–H2 interaction com- R Me the equilibrium lies completely binding and shorten dHH. The ligand plements classical Werner-type coordi- to the right, i.e., the complex is a ‘‘dihy- trans to H2 has a powerful influence: nation complexes where a ligand dride’’ (65). However, as shown above, strong ␲-acceptors such as CO (and also donates electron density through its H2 complexes are also stable with only strong ␴-donors such as H) greatly re- nonbonding electron pair(s) and ␲- small coligands L such as NH3 (Scheme duce backdonation and normally keep complexes such as olefin complexes in 1), in some cases with greatly elongated dHH Ͻ 0.9 Å. Thus one can favor a which electrons are donated from bond- dHH, two further paradigm shifts. This ␴-complex by placing the potential ␴- ing ␲-electrons (Scheme 2). It is re- led to extensive efforts to vary M, L, ligand trans to a strong ␲-acceptor. markable that the bonding electron pair and other factors to study stretching of Conversely, mild ␴-donors such as H2O in H2 can further interact with a metal the H–H bond. Within the large regime or ␲-donors such as Cl trans to H2 elon- center as strongly as a nonbonding pair of hundreds of LnM–H2 complexes, the gate dHH (0.96–1.34 Å), as dramatically in some cases. The resulting side-on reaction coordinate for the activation of demonstrated by the isomers in Scheme ␴ bonding in M-H2 and other -complexes H2 on a metal (Scheme 3) shows dHH 4 (70). The cis-dichloro complex is actu- is ‘‘nonclassical,’’ by analogy to the varying enormously, from 0.82 to 1.5 Å ally a ‘‘compressed trihydride’’ (dHH ϳ 3-center, 2-electron bonding in carboca- (3, 18, 25–31, 35–38, 44–60, 65–68). 1.5 Å) in solution, but in the solid state tions and diborane. The M center may This ‘‘arresting’’ of bond rupture along it is an elongated H2 complex (dHH ϭ ϩ be considered to be isolobal with H its entire reaction coordinate is unprece- 1.11 Å) due to Ir–Cl⅐⅐⅐H–Ir hydrogen ϩ and CH3 (61), mimicking carbocation dented. Although the dHH ranges shown bonding, illustrating the hypersensitivity chemistry; i.e., a ␴-complex such as are arbitrary, each category of com- of d to both intra- and intermolecular ϩ ϩ HH M –CH4 is related to CH5 , which is plexes has distinct properties. The dHH effects (71). Exceptions exist: the iso- viewed as a highly dynamic H2 complex is relatively short (0.8–1.0 Å), and H is mers of an ‘‘electron-poor’’ system, ϩ 2 of CH3 (62). H2 is thus a weak Lewis reversibly bound, in ‘‘true’’ H2 com- Cr(CO)4(PMe3)(H2), have similar JHD base that can bind to strong electro- plexes best exemplified by W(CO)3 (Ϸ34 Hz, hence dHH Ϸ 0.86 Å) whether philes, but transition metals are unique (PR ) (H ), much as in physisorbed H H is trans to CO or PMe (18). ␴ 3 2 2 2 2 3 in stabilizing H2 and other -bond com- where dHH is Ͻ0.8 Å. Elongated H2 At what point is the H–H bond ‘‘bro- plexes by ‘‘backdonation’’ of electrons complexes (dHH ϭ 1–1.3 Å) (29, 46, 66– ken’’? Theoretical analyses suggest 1.48 from a filled metal d orbital to the ␴* 69) were first clearly identified in 1991 Å, i.e., twice the normal length (72), but antibonding orbital of H2 (Scheme 2), a in ReH5(H2)(PR3)2 where neutron dif- little H–H bonding interaction remains critical interaction unavailable to main fraction showed a dHH of 1.357(7) Å for dHH Ͼ 1.1 Å (29). In certain ‘‘elon- group atoms (3, 4, 20). The backdona- (67). Complexes with dHH Ͼ 1.3 Å are gated’’ H2 complexes, e.g., [OsCl(H2) tion is analogous (4) to that in the ϩ now viewed as ‘‘compressed hydrides,’’ (dppe)2] , the energy barrier for stretch- Dewar–Chatt–Duncanson model (63, 64) with NMR features differing from elon- ing the H–H bond from 0.85 Å all of for ␲-complexes, e.g., M–ethylene. gated H2 complexes, e.g., JHD increases the way to 1.6 Å is calculated (29, 69) to A large variety of ␴-bonds X–H inter- act inter- or intramolecularly with metal centers (3, 42). In principle any X–Y bond can coordinate to a metal center, providing substituents at X and Y do not interfere. Backdonation of electrons fromMtoH2 (or to ␴* of any X–Y bond) is crucial not only in stabilizing ␴-bonding but also in activating the bond toward (3, 4, 20). If it becomes too strong, e.g., by increasing electron-donor strength of coligands on Scheme 3.

Kubas PNAS ͉ April 24, 2007 ͉ vol. 104 ͉ no. 17 ͉ 6903 Downloaded by guest on September 25, 2021 Eq. 6. Scheme 5.

complexes, a 74% reduction in J(13CH) available for other ␴-bond activations, be astonishingly low, Ϸ1 kcal/mol. The for low-temperature cyclopentane coor- e.g., C–H cleavage. of X–H H2 is highly delocalized: the H atoms dination in CpRe(CO)2(C5H10) (78), bonds through proton transfer to a basic undergo large amplitude vibrational mo- and 65% in J(11BH) in complexes of site on a cis ligand or to an external tion along the reaction coordinate for neutral borane ligands (79). JSiH in base is a crucial step in many industrial 2 H–H breaking. Remarkably, dHH is both M(␩ –Si–H) are normally closer to and biological processes involving direct ␩2 temperature and isotope dependent in those in OA products. –Ge–H bonds reaction of H2, silane, borane, and (pos- nϩ [CpM(diphosphine)(H2)] (M ϭ Ru, undergo OA much more easily than sibly) alkane ligands. ϭ Ir; n 1, 2) (73, 74). These phenomena Si–H, and in general, the ease of OA of H2 complexes can undergo heterolysis illustrate the highly dynamic behavior of H2 lies between that of germanes and in two distinct ways (Scheme 6). Intramo- coordinated H2 (40), which can exhibit silanes (80). Backdonation is critical: lecular heterolysis is extremely facile for quantum-mechanical phenomena such silanes bind more strongly than alkanes proton transfer to a cis ligand L (e.g., H as rotational tunneling (56) and ex- and cleave much like H2 because the or Cl) or to the counteranion of a cationic change coupling (75). M–H2 and other Si–H bond is a good acceptor whereas complex. The proton can also end up at a ␴-bond interactions are among the most C–H is not (the much higher energy of trans ligand (Eq. 6) (83). dynamic, complex, and enigmatic chemi- its ␴* orbital reduces interaction with M Intermolecular heterolysis involves pro- cal topologies known. The H2 ligand can d orbitals). However, the situation is tonation of an external base B, e.g., an Ϫ bind/dissociate, reversibly split to dihy- more complex than for H2 activation ether solvent, to give a metal hydride (H dride, rapidly rotate, and exchange with because substituents at C or Si alter fragment) and the conjugate acid of the cis hydrides, all on the same metal. Of- both electronics and sterics. base, HBϩ. This is the reverse of protona- ten these dynamics cannot be frozen out tion reactions used to synthesize H com- Reactivity of ␴-Complexes: Acidity 2 on the NMR time scale even at low T. plexes (all reactions in Scheme 6 can be and Heterolysis of X–H Bonds ϩ It is clear that H2 binding followed by reversible), and the [HB] formed can OA serves as a prototype for other Aside from loss of H2, reactions of relay the proton to internal or external ␴ -bond activation processes, e.g., C–H M–H2 are dominated by homolytic sites (base-assisted heterolysis). Crabtree and Si–H. Silanes (HnSiR4-n) bind in cleavage of H2 (OA) and heterolytic and Lavin (84) first demonstrated heterol- ␩2 -Si-H fashion (as in Eq. 2) (21–24, 42, cleavage, essentially deprotonation of ysisofH by showing that the H in ␩2 2 2 76). The -SiH4 structure in cis- bound H2 on electrophilic metal centers [IrIH(H )(LL)(PPh ) ]ϩ is deprotonated ␴ 2 3 2 Mo(CO)(SiH4)(Et2PC2H4PEt2)2, the (Scheme 5) (25). -Complexes have sev- by LiR in preference to the hydride li- first transition metal complex of SiH4, eral advantages in catalytic and other gand. A milder base, NEt3, was shown by exists in equilibrium with its OA tau- reactions. Foremost is that the formal Heinekey and Chinn (85) to more rapidly tomer, MoH(SiH3)(CO)(Et2PC2H4PEt2)2, oxidation state of M does not change on ␩2 2 deprotonate the -H2 tautomer in an analogous to that for the W(␩ -H2) sys- binding of H2, whereas formation of a ϩ equilibrium mixture of [CpRuH2(dmpe)] tem (Eq. 5), with similar structures and dihydride formally increases the metal ϩ and [CpRu(H2)(dmpe)] . The H2 ligand thermodynamic parameters (77). SiH4 oxidation state by two. H2 ligands can has greater kinetic acidity because depro- binding and Si–H cleavage directly also have far greater thermodynamic tonation of an H complex involves no model that believed to occur for CH and kinetic acidity than hydrides, which 2 4 change in coordination number or oxida- activation. Si–H distances in hydrosilane is important in the ability of acidic H 2 tion state. Thus, H gas can be turned complexes vary widely, analogous to H ligands to protonate substrates such as 2 2 into a strong acid: free H is an extremely complexes (3). A valuable yardstick for olefins and N . In heterolytic cleavage 2 2 weak acid [pK ϳ 35 in THF (86)], but measuring activation in M(␩2–X–H) (25, 40, 81, 82), the H ligand is depro- a 2 binding it to an electrophilic cationic bonds is the value of the NMR coupling tonated, and the remaining hydrogen metal increases the acidity spectacularly, constant JXH compared with that in the ligates to the metal as a hydride. Both up to 40 orders of magnitude. The pKa free ligand. There is typically a 50–80% pathways have been identified in cata- Ϫ ␩2 reduction in J in unstretched HD lytic hydrogenation and also may be can become as low as 6, i.e., –H2 can HD become more acidic than sulfuric acid as shown by Morris (25, 26, 82) and later Jia (36). Electron-deficient cationic H2 com- plexes with electron withdrawing ligands such as CO and short H–H bonds (Ͻ0.9 ϩ Å), i.e., [Re(H2)(CO)4(PR3)] (87) are among the most acidic. Positive charge increases acidity: W(CO)3(PCy3)2(H2)is deprotonated only by strong bases (88), but on oxidation to [W(CO)3(PCy3)2 ϩ (H2)] becomes acidic enough to proton- ate weakly basic ethers (89). Such ability Scheme 6. is relevant to processes such as ionic hy-

6904 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0609707104 Kubas Downloaded by guest on September 25, 2021 N O N C C C O C C O Eq. 7. Fe Fe S H S H drogenation and the function of metal- enge nucleophiles such as water or ab- S R loenzymes such as hydrogenases (H2ases). stract fluoride from anions such as Ϫ Complexes with H2 ligands are highly B(C6F5)4 (104). Similarly, a coordinated Sketch 2. dynamic, and cis interactions, which B–H bond in a BH3⅐PMe3 ligand in [Mn ϩ are hydrogen-bonding-like interactions (CO)4(PR3)(BH3⅐PMe3)] cleaves 2 Ϫ binding to a stepped Ni(510) surface between ␩ –H2 and a cis hydride ob- to give H (forming MnH(CO)4(PR3)) ϩ containing unsaturated sites was seen by servable in the solid state (3, 20, 42, 90), and ‘‘[BH2⅐PMe3] ’’ (105). facilitate solution exchange processes Can C–H bonds in alkanes bind to electron energy-loss spectroscopy (110) (Eq. 7). The intermediate is a ‘‘trihydro- electrophilic M to form a ␴-alkane com- and is the first step in hydriding other gen’’ complex (91, 92). Although not plex that can be split heterolytically? surfaces (111, 112). H2 also ligates at isolated, evidence exists for its interme- Proton transfer to a cis ligand (or an- low T in small clusters such as Cu3(H2) diacy in facile H-atom exchange in ) could take place followed by func- (113), Pd(H2) (114), and similar species (115). Oxides adsorb and activate H2, ReH2(H2)(CO)(PR3)3 (93), which can tionalization of the resultant methyl be exceedingly fast even at Ϫ140°C in complex. Increased acidity of C–H including Cr2O3, MgO, and ZnO even at hydrido(H ) complexes (94–99). The bonds in transient alkane complexes 25°C; some of these could involve mo- 2 ␩2 barrier for hydrogen exchange in analogous to that for coordinated H–H lecular binding. ( –H2)CrO2 has been i prepared by cocondensation of CrO2 IrClH2(H2)(P Pr3)2 is only 1.5 kcal/mol bonds may be important in alkane acti- even in the solid state (95, 96). vation such as conversion of methane to molecules with H2 in Ar at 11 K and Can direct transfer of hydrogens from methanol, a holy grail in chemistry well photoisomerized to HCrO(OH), ostensi- bly through H2 heterolysis (116). RuO2 an H2 ligand occur in catalytic hydro- addressed in this special feature and the (111) has also been found to bind H2 at genation? Although difficult to prove prolific work of Bercaw, Periana, and ␯ ϭ Ϫ1 ϭ conclusively, there is evidence in ionic Bergman. In 1965, Chatt discovered OA 85K( HH 2960 cm ; calcd dHH hydrogenation where an organometallic of an arene C–H bond to a metal com- 0.89 Å) (117) suggesting that, as for H2 hydride, e.g., CpMoH(CO) , plus a plex and in 1976 predicted that ‘‘in 25 on Ni surfaces, the binding of H2 is sim- 3 ilar to that in organometallics. Zeolites strong acid, e.g., HO SCF , reduce ke- years methane will be the most popular 3 3 can bind H (118, 119), notably to the tones (100, 101). An acidic H complex ligand in coordination chemistry,’’ as 2 2 extraframework iron in Fe–ZSM5 at 110 is involved in proton transfer (Scheme noted by Shilov (106). As can be seen, K. Research at the interface between 7). An impressive example of catalysis this prediction has become true. As in heterogeneous and homogeneous cataly- employing heterolysis of H is the asym- H activation, alkane ␴-complexes 2 2 sis (41) includes employing H interac- metric hydrogenation of ketones to alco- should be intermediates, astonishingly 2 tions as probes for the catalytic sites hols catalyzed by the system even in reaction media as harsh as sulfu- in both regimes (120–122). An elegant of Nobel Laureate Ryoji Noyori (102, ric acid at 200°C in PtII-catalyzed meth- example is the demonstration that Ir- 103). Other ␴-bonds can be cleaved het- ane to methanol conversions (107, 108), (CO)Cl(PPh3)2 catalyzes hydrogenation erolytically, particularly on electrophilic despite the weak binding energy of CH4 Ϸ of unsaturated compounds both in solu- metals (3, 40, 42). For coordinated Si–H to metals [ 10 kcal/mol (109)]. tion and solid state through an H com- bonds, the bond becomes polarized Molecular binding and heterolysis of 2 ϩ Ϫ plex (123). Si(␦ )–H(␦ ), i.e., the Si becomes posi- H2 on metal surfaces and small metal

tively charged (Scheme 8). Very reactive clusters is rarely observed because Activation of H2 on Biological and silylium are eliminated; they scav- formation of hydrides is favored. H2 Nonmetal Systems

H2ases are redox enzymes in microor- ganisms that catalyze H p 2Hϩ ϩ 2eϪ R 2 + O to either use H2 as an energy source or H R R + dispose of excess electrons as H2 (124– MH + HA M OH + MH R – 127). Biologically unprecedented CO H – A A and CN ligands are present in the dinuclear active site of iron-only H2ases (128) that are remarkably organometal-

+ – lic-like and have been extensively mod- R2CHOH + MA [M(OHCHR2)] A eled for biomimetic H2 production (126, Scheme 7. 127, 129–132) (see Sketch 2). This site presumably transiently binds and heterolytically splits H2, most likely at a site trans to bridging CO, where a proton transfers to a thiolate ligand as in Eq. 6 or other Lewis-basic site (127). Such heterolysis has recently been shown to occur on a mononuclear Fe complex with a pendant nitrogen base (132). Nature apparently designed these enzymes billions of years ago to use the Scheme 8. CO ligand, whose strong trans influence

Kubas PNAS ͉ April 24, 2007 ͉ vol. 104 ͉ no. 17 ͉ 6905 Downloaded by guest on September 25, 2021 ϩ F F H F F H H2 Storage and Production: A Glance [M(H2)n] have a fleeting gas phase ex- H2 δ δ+ + – to the Future istence (142), but isolation in condensed – P B P B H2 is a fuel of the future, but vexing phases will be problematic. F F F F challenges exist. Materials for H2 stor- Production of H2 fuel from water by age are difficult to design because, al- means of solar energy is of high interest Scheme 9. though H2 can readily be extruded from (143). Catalysis may involve H2 com- a variety of compounds, it can be diffi- plexes at least as intermediates, and H2 complexes have been implicated in solar favors reversible H binding and heterol- cult to add back. The materials also 2 must be light and contain Ͼ6% by energy conversion schemes based on ysis (3, 40). An H2 complex of a H-ase weight H2, reducing prospects for known photoreduction of water (144). Industri- model, [Ru2(␮-H)(␮-S2C3H6)2(H2)(CO)3 ϩ facile reversible systems such as ally important water gas shift and (PCy3)2] , is known, albeit with Ru in- related H -producing reactions undoubt- metal–H2 or hydride complexes. Amine 2 stead of Fe (133). edly proceed through transient H com- borane, H3NBH3, is a popular candidate 2 H2 can also be activated at nonmetals, and also combines both Lewis acidic (B) plexes (145). Biomimetic H2 production, e.g., the bridging sulfides in Cp2Mo2S4 particularly solar driven (photocatalysis), that react with H to form SH ligands and basic (N) centers. Here, however, 2 is also a challenge and may take a cue perhaps via a 4-center S H transition these centers are directly bonded, 2 2 from models of the active site of H ase state (134). Metal-free hydrogenation of whereas the acidic and basic sites are 2 coupled with models of nature’s photo- ketones on strong bases such as t-BuOK separated by linkers in the phosphine- systems (129–131, 143). Here the forma- occurs under harsh conditions, appar- borane in Scheme 9. The metal-free as- tion of H–H bonds from protons and ently through base-assisted heterolysis of pect is relevant because precious metals electrons, the microscopic reverse of H H (135, 136). Thus, H is a very weak such as platinum are often used in catal- 2 2 2 heterolysis, will be crucial in leading to acceptor (Lewis acid) through electron ysis and can be environmentally un- formation of H and is very rapid at the donation to its ␴* orbital and can inter- friendly as well as costly or in short sup- 2 Fe sites in H -ases. Coupling model cat- act with the O in alkoxide or metal ox- ply. Materials such as metal-organic 2 alysts with photochemical water splitting ides and undergo heterolysis (3). Signifi- frameworks (MOFS) (138–140) are now will require fine-tuning of electrochemi- cantly, the first example of reversible being examined for H2 storage and have splitting of H on a nonmetal center has huge surface area capable of binding cal potentials for tandem catalysis 2 schemes. been found (137). The phosphine bo- large numbers of H2 molecules. Neutron rane in Scheme 9 has a strong Lewis scattering studies by Eckert are critical I am grateful to funding by the Department acidic center (boron) linked to a Lewis in determining whether H2 binds to un- of Energy, Basic Energy Sciences, Chemical basic site (phosphorus). It is likely that saturated metal centers as in organome- Sciences that allowed me to carry out the H heterolysis takes place at boron tallics and/or is physisorbed in the 2 basic research leading to the discovery of H2 where proton transfer from an H2-like framework. Calculations indicate com- complexes and Los Alamos National Labora- complex to the basic phosphorus site plexes with multiple H2, i.e., Cr(H2)6 tory for Laboratory Directed Research and occurs to form a phosphenium–borate. may be stable (141), and species such as Development funding.

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