1-(4-Chlorophenyl)imidazole-2-thione (HtimAr) and its derivative tris(mercaptoimidazolyl)borate [BH(timAr)3]− of ruthenium complexes

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry

N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and low-coordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element-metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.

5-Mercaptotetrazolyl-derived metallaboratranes

The reactions of 1-tert butyl-5-thiotetrazole (HtttBu) with Na[BH4] provides Na[H2B(tttBu)2] (Na[1]) or Na[HB(tttBu)3] (Na[2]) in refluxing THF or xylene, respectively. Treating [RuCl(Ph)(CO)(PPh3)2] with Na[2] affords the triply-buttressed ruthenaboratrane [Ru(CO)(PPh3){κ4-B,S,S',S''-B(tttBu)3}] (5) whilst Na[1] with [IrCl(CO)(PPh3)2] or [RuCl(Ph)(CO)(PPh3)2] provides rare examples of doubly-buttressed metallaboratranes [IrH(CO)(PPh3){κ3-B,S,S'-BH(tttBu)2}] (7) and [Ru(CO)(PPh3)2{κ3-B,S,S'-BH(tttBu)2}] (12), the latter via the isolable thiotetrazoylborate complex [Ru(Ph)(CO)(PPh3){κ3-H,S,S'-H2B(tttBu)2}] (11).

Synthesis and ligand substitution reactions of κ4-B,S,S′,S′′-ruthenaboratranes

Ruthenaboratranes of the form [Ru(CO)L{κ⁴-B(mt)₃}] (mt = N-methimazolyl) arise via substitution of the PPh₃ ligand in [Ru(CO)(PPh₃){κ⁴-B(mt)₃}] by L (L = PMe₂Ph, PMe₃, P(OMe)₃, P(OEt)₃, P(OPh)₃) or reactions of [RuCl(R)(CO)L_n] (R = Ph, CHCHPh; n = 2, L = PCy₃; n = 3, L = P(OMe)₃, PMe₂Ph) with Na[HB(mt)₃].

Methimazolyl based diptych bicyclo-[3.3.0]-ruthenaboratranes

The reactions of [RuCl(R)(CO)(PPh₃)₂] (R = CHCHPh, Ph) with Na[H₂B(mt)₂] (mt = N-methyl-2-mercaptoimidazolyl) transiently provide [Ru(R)(CO)(PPh₃){κ³-H,S,S′-H₂B(mt)₂}] which each evolve to the ruthenaboratrane [Ru(CO)(PPh₃)₂{κ³-B,S,S′-BH(mt)₂}](Ru→B)⁸.

Dihydrobis(methimazolyl)borato complexes of ruthenium and osmium

The reaction of Na[H₂B(mt)₂] (mt = 2-mercapto, 3-methylimidazol-1-yl, methimazolyl) with [Ru(X)Cl(CO)(PPh₃)_n] (n= 3 X = H;n= 2 BO₂C₆H₄, SiCl₃, SiMe₃) affords the complexes [Ru(X)(CO)(PPh₃){κ²-H,S,S′-H₂B(mt)₂}].