Dihydrogen activation by metal alkyl complexes via s-bond metathesis is a facile non-redox reaction and a common method for the preparation of rare-earth [1] and early-transition-metal hydrido complexes. [2] The intramolecular CÀH bond activation has been reported for metal alkyl or hydride complexes to give so-called "tucked-in" or "tucked-over" cyclometalated complexes with concomitant liberation of alkane or dihydrogen, [3] predominantly in metallocenes of early transition metals [2b, 4] and f-block elements. [5] The metallocene hydrides of zirconium [6] and samarium [5d] and tris-(amido) actinide hydrides [7] reversibly form the hydrido complex and the cyclometalated complexes by addition and removal of dihydrogen.We have reported a bimetallic trivalent rare-earth-metal complex with two bridging hydrido ligands that adds dihydrogen across a metal-carbon bond and a metal-metal axis. [8] The reverse liberation of dihydrogen by CÀH bond activation was not observed. [9] We have now obtained a rare-earth-metal hydrido complex capable of reversibly binding and releasing dihydrogen under mild conditions. No change in the oxidation state of the metals is involved, as addition and release occur by s-bond metathesis involving a metal-carbon bond. Notably, hydrogenolysis in the solid state was also observed in a single-crystal to single-crystal transformation.The cationic dialkyl lutetium complex [Lu(CH 2 SiMe 3 ) 2 -(thf) 3 ][A] (A = B{3,5-C 6 H 3 (CF 3 ) 2 } 4 ) [10] was treated with the neutral NNNN-type macrocyclic ligand 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane (Me 4 TACD = Me 4 [12]aneN 4 ) to give the THF-free dialkyl complex [(Me 4 TACD)Lu-(CH 2 SiMe 3 ) 2 ][A] (1-Lu) in quantitative yield (Scheme 1). Scheme 1. CÀH bond activation in cationic bis(alkyl) complex 1-Lu to give complex 2-Lu.Scheme 2. Preparation of dicationic trihydrido complex 3-Lu.

We developed a salt-free reduction of WCl6 using 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (MBTCD) in toluene to give a low-valent trinulcear tungsten complex involving W(II) and W(III) centers, while in the presence of redox active ligands such as α-diketone and α-diimine the same reduction produced W(IV) complexes with the corresponding redox-active ligands, (α-diketone)WCl4 and (α-diimine)WCl4. A W(VI) complex with two α-diketone ligands, (α-diketone)2WCl2, was found to be synthetically equivalent to low-valent W(IV) species that trapped azopyridine to give (α-diketone)WCl2(azopyridine).

Tris(trimethylsilylmethyl) complexes of yttrium and lutetium [LnR(3)(THF)(2)] (R = CH(2)SiMe(3)) were treated with sterically bulky N-heterocyclic carbenes (NHC) 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) and 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes). IPr gave labile mono-adducts [LnR(3)(NHC)], isolated as thermally robust crystals and fully characterized by NMR spectroscopy and X-ray diffraction. IMes gave a similar lutetium mono-adduct [LuR(3)(IMes)] with the lutetium alkyl [LuR(3)(THF)(2)], whereas the yttrium alkyl [YR(3)(THF)(2)] resulted in the formation of an ortho-metalated product. This compound, isolated as a crystalline bis(THF) adduct, contains a strained six-membered chelate ring that has been formed by the C-H bond activation of one of the ortho-methyl groups of the mesityl group. In contrast [LuR(3)(IMes)] only slowly underwent a similar C-H bond activation.

The dinuclear lutetium dihydride dication supported by metalated tripodal ligands undergoes facile hydrogenolysis with H(2) to form a trihydride dication. Molecular orbital analysis shows that the LUMO is a bonding Lu···Lu orbital that is poised to activate dihydrogen.

Molecular hydrides of the rare-earth metals play an important role as homogeneous catalysts and as counterparts of solid-state interstitial hydrides. Structurally well-characterized non-metallocene-type hydride complexes allow the study of elementary reactions that occur at rare-earth-metal centers and of catalytic reactions involving bonds between rare-earth metals and hydrides. In addition to neutral hydrides, cationic derivatives have now become available.