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Lanthanide complexes have attracted a widespread attention due to their structural diversity, as well as multifunctional and tunable properties. The development of lanthanide based functional materials has often relied on the design of the secondary coordination sphere of the corresponding lanthanide complexes. For instance, usually simple lanthanide salts (solvento complexes) do not catalyze effectively organic reactions or provide low yield of the expected product, whereas the presence of a suitable organic ligand with a noncovalent bond donor or acceptor centre (secondary coordination sphere) modifies the symmetry around the metal centre in lanthanide complexes which then successfully can act as catalysts in both homogenous and heterogenous catalysis. In this minireview, we discuss several relevant examples, based on X-ray crystal structure analyses, in which the hydrogen, halogen, chalcogen, pnictogen, tetrel and rare-earth bonds, as well as cation-π, anion-π, lone pair-π, π-π and pancake interactions, are used as a synthon in the decoration of the secondary coordination sphere of lanthanide complexes.

Two known lanthanide complexes, [KLa(HL 1 ) 2 {(CH 3 ) 2 NCHO} 2 (H 2 O) 3 ] ( 1 ) and [Sm(H 2 O) 9 ]( E -H 2 L 2 ) 3 ·2H 2 O ( 2 ) were prepared by reaction of lanthanum(III) and samarium(III) nitrates with potassium 3-(2-(2,4-dioxopentan-3-ylidene)hydrazinyl)-2-hydroxy-5-nitrobenzenesulfonate (KH 2 L 1 ) and potassium ( E,Z )-5-chloro-3-(2-(1,3-dioxo-1-phenylbutan-2-ylidene)hydrazinyl)-2-hydroxybenzenesulfonate (KH 2 L 2 ), respectively. Catalytic activities of 1 and 2 were evaluated in the cyanosilylation of aldehydes with trimethylsilyl cyanide in different solvents such as tetrahydrofuran, dichloromethane or methanol. Complex 1 was found to be efficient catalyst for the cyanosilylation reaction in methanol medium at room temperature, producing cyanohydrin trimethylsilyl ethers with good yields (76–99%).

The new mononuclear nickel(II) complexes [Ni(HL 1 )(H 2 O) 2 (CH 3 OH)] ( 1 , NaH 2 L 1 = sodium ( Z )-2-(2-(1,3-dioxo-1-(phenylamino)butan-2-ylidene)hydrazinyl)benzenesulfonate) and [Ni(H 2 L 2 ) 2 (H 2 O) 4 ] [(CH 3 ) 2 S O) 2 ] ( 2 , H 3 L 2 = (Z)-2-(2-(1,3-dioxo-1-(phenylamino)butan-2-ylidene)hydrazinyl)benzoic acid) were synthesized and characterized by IR and ESI-MS spectroscopies, elemental and X-ray crystal structural analyses. Both compounds 1 and 2 act as homogenous catalysts for the diastereoselective nitroaldol (Henry) reaction of aliphatic and aromatic aldehydes with nitroethane in different solvents such as acetonitrile, methanol or water. Complex 1 was found to be the more efficient catalyst for the Henry reaction in aqueous medium, providing β-nitroalcohols with good yields (67–82%) and diastereoselectivities ( syn/anti 59:41–70:30).

A series of known metal complexes, [LaL2(NO3)(H2O)2]·5H2O (1), [Zn(μ-L)(NO3)(H2O)]n (2), {[Cd2(μ-HL)(μ-L)(NO3)3(H2O)]·H2O}n (3) and [Hg(μ-L)(N3)]n·nH2O (4), were synthesized from the corresponding metal salts and 2-pyridinecarboxaldehyde isonicotinoylhydrazone (HL), and demonstrated high activity and selectivity in the cyanosilylation of aldehydes. Although all the complexes are derived from the same HL hydrazone ligand, the catalytic activity of 1–4 is different and relates to the metal center and coordination environment, whereas complex 1 being the most effective homogeneous Lewis acid-base catalyst towards the synthesis of cyanohydrin trimethylsilyl ethers from both aliphatic and aromatic aldehydes with yields up to 96.3 % in 6 h in methanol solution and at room temperature. According to the proposed mechanism, both coordination and noncovalent interactions (hydrogen and tetrel bonds) play a significant role and account for the higher catalytic activity of complex 1.

Two known organotin(IV) complexes, [Sn(C6H5)3HL1] (1) and [Sn(C2H5)2(1κO,2κO-H3L2) (1κO2-H3L2) (μ3-O)]2 (2), were prepared upon interaction of Ph3SnCl and Et2SnO with 2-(2-(2,4-dioxopentan-3-ylidene)hydrazinyl)benzoic acid (H2L1) and 2-(2-(2,4,6-trioxotetrahydropyrimidin-5(2H)-ylidene) hydrazinyl)benzoic acid (H4L2), respectively, in toluene solution. Both complexes are applied as catalysts in cyanosilylation of aldehydes. Complex 2 shows a high catalytic activity in the cyanosilylation reaction of a variety of aliphatic and aromatic aldehydes with trimethylsilyl cyanide yielding to the corresponding cyanohydrin trimethylsilyl ethers (yields up to 99%) in methanol and at room temperature.

Two complexes [KLa(HL(1))2{(CH3)2NCHO}2(H2O)3] (1) and [Sm(H2O)9](E-H2L(2))3·2H2O (2) were synthesized by the reaction of lanthanum(III) and samarium(III) nitrates with potassium 3-(2-(2,4-dioxopentan-3-ylidene)hydrazinyl)-2-hydroxy-5-nitrobenzenesulfonate (KH2L(1)) and potassium (E,Z)-5-chloro-3-(2-(1,3-dioxo-1-phenylbutan-2-ylidene)hydrazinyl)-2-hydroxybenzenesulfonate (KH2L(2)), respectively. Both complexes were fully characterized by elemental analysis, IR, (1)H and (13)C NMR spectroscopy, ESI-MS and single-crystal X-ray diffraction. Cooperative E,Z→E isomerization of KH2L(2), induced by resonance assisted hydrogen bonding and ionic interactions, occurs upon the interaction with Sm(III). Catalytic activities of KH2L(1,2) and their lanthanide derivatives were evaluated in the Henry reaction of nitroethane with a variety of aromatic and aliphatic aldehydes. Good yields (up to 91%) and diastereoselectivities (syn/anti 3 : 1) were observed in the reactions catalyzed by 1 in water.