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.
Beyond Cp: Molecular hydrides of rare‐earth metals play an important role as homogeneous catalysts and as models of solid‐state interstitial hydrides. This Review provides an overview of rare‐earth‐metal hydride complexes(without cyclopentadienyl ligands), with emphasis on the structural motifs and the effect of cationic charges on reactivity.
The hydride ligand in the cationic calcium hydride supported by a NNNN-type macrocycle, (Me4TACD)2Ca2(μ-H)2(THF)BAr42 (1; Me4TACD = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane; THF = ...tetrahydrofuran; BAr4 = B(C6H3-3,5-Me2)4), shows, in addition to its Brönsted basicity toward weak acids, a pronounced nucleophilicity resulting in nucleophilic substitution or insertion (addition) at a silicon or sp2 carbon center. Terminal acetylenes RCCH (R = SiMe3, cyclopropyl) as well as 1,4-diphenylbutadiene were deprotonated by 1 to give dinuclear complexes (Me4TACD)2Ca2(μ-CCR)2BAr42 (2a, R = SiMe3; 2b, R = cyclopropyl) and (Me4TACD)2Ca2(μ2-η4-1,4-Ph2C4H2)BAr42 (3) with H2 evolution. The addition reaction with BH3(THF) gave a tetrahydridoborate complex, (Me4TACD)Ca(BH4)(THF)2BAr4 (4), with κ2-H2BH2 coordination in the solid state, suggesting a pronounced Lewis acidic calcium center. The behavior resulting from both Lewis acidity and hydricity becomes apparent in the nucleophilic substitution of fluorobenzene by 1 to give benzene and the dimeric fluoride complex (Me4TACD)2Ca2(μ-F)2(THF)BAr42·2.5THF (5). Analogous nucleophilic substitution reaction is observed for heterofunctionalized organosilanes XSiR3 X = I, N(SiHMe2)2, N3; R = Me3 or HMe2, which resulted in the formation of calcium complexes (Me4TACD)Ca(X)(THF) n BAr4 (6–8) containing an X ligand along with hydrosilane HSiR3. An insertion reaction by 1 was observed with CO2 and CO to give dinuclear formato complex (Me4TACD)2Ca2(μ-OCHO)2BAr42 (9) and cis-enediolato complex (Me4TACD)2Ca2(μ-OCHCHO)BAr42·3.5THF (10), respectively. The latter is believed to have been formed as a result of the dimerization of an initially generated formyl or oxymethylene complex, (Me4TACD)Ca(OCH)+.
Hydrogenolysis of bis(triphenylsilyl)calcium containing the neutral NNNN‐type macrocyclic amine ligand Me4TACD Ca(Me4TACD)(SiPh3)2 (2), gave the cationic dinuclear calcium hydride ...Ca2H3(Me4TACD)2(SiPh3) (3), characterized by NMR spectroscopy, single‐crystal X‐ray analysis, and DFT calculations. Compound 3 reacted with deuterium to give the deuteride D3‐3.
H,H,H‐held together: Hydrogenolysis of the bis(triphenylsilyl)calcium complex Ca(Me4TACD)(SiPh3)2 gave the cationic dinuclear calcium hydride Ca2H3(Me4TACD)2(SiPh3) that undergoes fast deuteration.
The trinuclear cationic zinc hydride cluster (IMes)3Zn3H4(THF)(BPh4)2 (1) was obtained either by protonation of the neutral zinc dihydride (IMes)ZnH22 with a Brønsted acid or by addition of the ...putative zinc dication (IMes)Zn(THF)2+. A triply bridged thiophenolato complex 2 was formed upon oxidation of 1 with PhSSPh. Protonolysis of 1 by methanol or water gave the corresponding trinuclear dicationic derivatives. At ambient temperature, 1 catalyzed the hydrosilylation of aldehydes, ketones, and nitriles. Carbon dioxide was also hydrosilylated under forcing conditions when using (EtO)3SiH, giving silylformate as the main product.
Competition for transition metals: A cationic trinuclear zinc hydride cluster with a Zn3H4 core efficiently catalyzes the hydrosilylation of aldehydes, ketones, and nitriles, and notably also carbon dioxide.
The ring-opening polymerization of l-lactide initiated by rare earth metal silylamido complexes Ln(OSSO){N(SiHMe2)2}(THF) (1−3: Ln = Y, Lu; OSSO = 1, ω-dithiaalkanediyl-bridged bisphenolato) was ...studied. MALDI-TOF mass spectrometry and 1H NMR spectroscopy suggested that the polymerization proceeded via a conventional coordination−insertion mechanism involving silylamide ligand as the initiating group and the cleavage of acyl−oxygen bond of the monomer. A two-stage linear relationship between ln(LA0/LA t ) and the polymerization time was observed for the yttrium complex Y(pdtbp){N(SiHMe2)2}(THF) (pdtbp = 1,5-dithiapentanediyl-bis{4,6-di-tert-butylphenolato}, 3). In both stages, the polymerization showed first-order kinetics for the monomer concentration. The first-order dependency of the initiator concentration was only observed when the monomer conversion to PLA was less than 50−60%. The aggregation of the active growing polymer chain into dimeric structure occurred in the second stage. In contrast, the in situ generated alkoxide initiator Ln(OSSO){N(SiHMe2)2}(THF)/ i PrOH showed a different behavior. When 3 was reacted with 2-propanol in 1:2 ratio, the in situ generated alkoxides initiated the living polymerization of l-lactide. Neither aggregation nor intramolecular transesterification was observed over the entire conversion range. Polylactides with controlled molecular parameters (M n, end groups) and low polydispersities were formed as a result of fast alkoxide/alcohol exchange.
A new family of heterometallic catalysts based on trimetalated macrocyclic tris(salen) ligands and rare‐earth metals was prepared and structurally characterized. The LaZn3 system containing anionic ...ligands such as acetate plays a critical role in catalyzing the alternating copolymerization of cyclohexene oxide (CHO) and CO2 with a high proportion of carbonate linkages. Among the lanthanide metals, the CeZn3 system exhibits high catalytic activity with a turnover frequency (TOF) of over 370 h−1. NMR analysis of the complex and end‐group analysis of the polymer suggest that the acetate ligands are rapidly exchanged, not only among coordinated acetates, but also between coordinated acetates and added carboxylate anions. These unique properties make this the first example of telomerization for the copolymerization of CHO and CO2.
Homogeneous heterometallic complexes based on the trizincated macrocycle trisaloph and a rare‐earth metal showed high catalytic activity for the alternating copolymerization of cyclohexene oxide and CO2 with a high proportion of carbonate repeat units. The carboxylate anion of the ammonium salt initiates the telomerization, providing the polycarbonate with the corresponding carboxylate.
Sequential tetradentate dianionic thio-imine diphenolate ligands featuring an ortho-phenylene core and their zirconium complexes are described for the first time. Ligands that include different ...combinations of bulky-alkyl groups and halo groups on the two phenol arms were prepared by a substitution/condensation reaction sequence. An unexpected fac-fac wrapping mode was found in the solid state for the ligands in the octahedral {ONSO}Zr(OtBu)2 complexes. The complexes were all fluxional, and the barrier for enantiomer interconversion was found to depend on the phenolate substituents. The complexes were found to catalyze the polymerization of rac-lactide to poly(lactic acid) in solution with polymer tacticities varying from heterotactic to atactic which showed correlation to the nature of phenolate substituents but not to the degree of complex fluxionality.
Triphenylborane (BPh3) in highly polar, aprotic solvents catalyzes hydrosilylation of CO2 effectively under mild conditions to provide silyl formates with high chemoselectivity (>95 %) and without ...over‐reduction. This system also promotes reductive hydrosilylation of tertiary amides as well as dehydrogenative coupling of silane with alcohols.
Polar power: Triphenylborane in highly polar, aprotic solvents catalyzes the hydrosilylation of CO2 under mild conditions to silyl formates with remarkable selectivity (>95 %) and without over‐reductions. This system also promotes reductive hydrosilylation of tertiary amides and dehydrogenative coupling of silane with alcohols.
Alkali metal hydridotriphenylborates (L1)MHBPh3 (L1=Me6TREN; M=Li, Na, K) chemoselectively catalyze the hydroboration of carbonyls and CO2, with lithium being the most active system. A new series of ...complexes (L2)MHBPh3 M=Li (1), Na (2), K (3) featuring the cyclen‐derived macrocyclic polyamine Me4TACD (L2) were synthesized in a “one‐pot” fashion and fully characterized including X‐ray crystallography. In the crystal, 1–3 exhibit wide variation in metal coordination of the HBPh3− anion from lithium to potassium. The structures differ from those in (L1)MHBPh3. Effects of coordination of L1, L2, and other N‐ and O‐donor multidentate ligands on Li(HBPh3) were used to rationalize the catalytic activity in carbonyl hydroboration.
Light metal: Comparison of ligands (L) on the catalytic carbonyl hydroboration by (L)MHBPh3 (M=Li, Na, K) confirms the unique combination of Me6TREN and lithium for high activity.
Potassium silanide KSiH3∞ contains 4.2 wt % of hydrogen and has been intensely studied as hydrogen storage material. The macrocyclic ligand Me4TACD ...(1,4,7,10‐tetramethyl‐1,4,7,10‐tetraaminocyclododecane, L) stabilizes the full range of triphenylsilyl complexes (L)MSiPh3n (M=Li–Cs), which react with H2 or PhSiH3 to form molecular (L)MSiH3n that can be isolated in soluble form and fully characterized.
Silanides: The macrocyclic polyamine ligand Me4TACD (L) stabilizes the full range of alkali metal triphenylsilanides (L)MSiPh3n (M=Li–Cs) that have been studied in solution and in the solid state. They react with H2 or PhSiH3 to give molecular trihydridosilanides (L)MSiH3 that are soluble in THF and can be regarded as model complexes for the hydrogen‐storage material MSiH3∞.
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