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The review covers the mechanism insights and recent achievements in catalyst design for oxidative coupling of benzoic acids and alkynes. This reaction is a straightforward effective approach to synthesis of isocoumarins or naphthalenes. The influence of catalyst nature and reaction conditions on the selectivity is also discussed. This contribution aims to focus attention of organic chemists on the possibility to use a wide range of catalysts, without being limited by only conventional complexes.

A series of new osmacarborane complexes were synthesized by treatment of the 11-vertex carborane [closo-СВ10Н11]–[Me4N]+ (1) with tris(triphenylphosphine)osmiumdichloride OsCl2(PPh3)3 (2). We managed to control the process of the carborane framework transformation {closo-CB10} → {hypercloso-OsCB10} → {hypercloso-OsCB9} → {hypercloso-OsCB8}, and each stage of this polyhedral carborane contraction was experimentally confirmed. The isolated clusters are electron-deficient hypercloso-osmacarboranes with 2n skeletal electrons. The structures of two “anti-Wade” complexes (10- and 11-vertex osmacarboranes) were confirmed by X-ray diffraction.

The heating of the isomeric [1-(η6-1,4-Me2C6H4)-3-Cl-isonido-1,2,4-RuC2B8H9] (1) or [1-(η6-1,4-Me2C6H4)-6-Cl-isonido-1,2,4-RuC2B8H9] (2) complexes (i) in a 1,4-Me2C6H4 solution at 85 °C as well as (ii) in a melt at 165 °C afforded mixtures of isomeric isonido- and closo-ruthenacarboranes, which differ in the positions of the chlorine substituent and the carbon atoms in the cage ligand. Structures of all new isomeric complexes have been established on the basis of analytical and multinuclear NMR data and were characterized by single-crystal X-ray diffraction studies. Also presented are the results of the quantum chemical calculations for the ground and the transition states of the reaction (i), which proved to be basis for the suggestion of interconversion scheme for these isomers.

The monocarbon carborane [Cs][nido-7-CB(10)H(13)] reacts with the 16-electron [RuCl(2)(PPh(3))(3)] in a solution of benzene/methanol in the presence of N,N,N',N'-tetramethylnaphthalene-1,8-diamine as the base to give a series of 12-vertex monocarbon arene-biruthenacarborane complexes of two types: [closo-2-[7,11-exo-RuClPPh(3)(mu,eta(6)-C(6)H(5)PPh(2))]-7,11-(mu-H)(2)-2,1-RuCB(10)H(8)R] (5, R = H; 6, R = 6-MeO; 7, R = 3-MeO) and [closo-2-(eta(6)-C(6)H(6))-10,11,12-[exo-RuCl(PPh(3))(2)]-10,11,12-(mu-H)(3)-2,1-RuCB(10)H(7)R(1)] (8a, R(1) = 6-MeO; 8b, R(1) = 3-MeO, inseparable mixture of isomers) along with trace amounts of 10-vertex mononuclear hypercloso/isocloso-type complexes [2,2-(PPh(3))(2)-2-H-3,9-(MeO)(2)-2,1-RuCB(8)H(7)] (9) and [2,5-(Ph(3)P)-2-Cl-2-H-3,9-(MeO)(2)-2,1-RuCB(8)H(6)] (10). Binuclear ruthenacarborane clusters of both series were characterized by a combination of analytical and multinuclear NMR spectroscopic data and by single-crystal X-ray diffraction studies of three selected complexes, 6-8. In solution, isomers 8a,b have been shown to undergo the isomerization process through the scrambling of the exo-[RuCl(PPh(3))(2)] fragment about two adjacent triangular cage boron faces B(7)B(11)B(12) and B(8)B(9)B(12).

The presence of two low-frequency large-amplitude modes in the vibrational spectrum, experimental and computed data on B–B interatomic distances and the results of a topological analysis of the electron density distribution in the molecule of an 11-vertex monocarbaborate [2-CB10H11]− lead to the conclusion that this polyhedron is not rigid, missing at least four double-center B–B bonds and thus being formed not only by triangular faces. Hence, this carbaborane does not have the closo-structure traditionally ascribed to it on formal [2n+2]-electron count grounds.

Reactions of [OsCl2(PPh3)3] 1 with the [NEt4]+ salts of the [6-Ph-nido-6-CB9H11]− anion 2 and the [arachno-6-CB9H14]− anion 3 each proceed selectively in methanol solution with the elimination of one boron-containing fragment from the starting cluster polyhedron, yielding together a short series of three novel 10-vertex hydridoosmamonocarbaboranes of isocloso {OsCB8} cluster geometry.

Heating of 1,1,3-(PPh3)3-1-H-1,2,4-RuC2B8H9 with 2-(hydroxymethyl)bicyclo[2.2.1]hepta-2,5-diene in arene solvents (benzene, toluene, or mesitylene) unexpectedly afforded the ruthenium arene complexes 1-(η6-arene)-3-(C7H9CH2O)-isonido-1,2,4-RuC2B8H9 containing the nortricyclene fragment in the carborane ligand.

A series of arene-ruthenium 11-vertex complexes 3-Cl-1-(η6-arene)-isonido-1,2,4-RuC2B8H9 (4a-4h) and 6-Cl-1-(η6-arene)-isonido-1,2,4-RuC2B8H9 (5a, 5b, 5f, 5g) (arene = p-xylene (a), mesitylene (b), p-cymene (c), benzene (d), toluene (e), hexamethylbenzene (f), 1,2,4,5-tetramethylbenzene (g), 1,2,4-trimethylbenzene (h)) have been obtained as isomeric mixtures 4a, 4b, 4f, 4g and 5a, 5b, 5f, 5g or, predominantly, as single isomers 4c-4e, 4h by the treatment of nido-5,6-C2B8H12 (1) with [RuCl2(PPh3)3] (2), either in the corresponding arene solvents at 80-85 °C (in the case of 4a-4e and 5a, 5b) or in the arene/1,2-dichloroethane solution under reflux (in the case of 4f-4h and 5f, 5g) in the presence of N,N,N',N'-tetramethylnaphthalene-1,8-diamine. The compounds prepared were characterized by combination of analytical, IR and multinuclear NMR data, including a crystallographic study of 4b. A possible pathway for the formation of isomeric complexes 4 and 5 is discussed.

RuCl2(PPh3)3 (1), when heated with an equimolar amount of nido-5,6-C2B8H12 (2) in toluene for a few hours, effected rather unexpected polyhedral contraction of 2 to give the new half-sandwich small ruthenacarborane closo-C2B4H6Ru(PPh3)2HCl (3) in moderate yield along with minor amounts of the two isomeric 11-vertex ruthenacarboranes [3-Cl-1-(η6-C6H5Me)-1,2,4-RuC2B8H9] (4) and [6-Cl-1-(η6-C6H5Me)-1,2,4-RuC2B8H9] (5) with open MC2B8 cluster structure. These new complexes were characterized by 1H, 13C, and 11B NMR spectroscopy and, in the case of 3 and 5, also by an X-ray crystal structure analysis.

The monocarbon carborane [Cs][nido-7-CB(10)H(13)] reacts with the 16-electron [RuCl(2)(PPh(3))(3)] in a solution of benzene/methanol in the presence of N,N,N',N'-tetramethylnaphthalene-1,8-diamine as the base to give a series of 12-vertex monocarbon arene-biruthenacarborane complexes of two types: [closo-2-[7,11-exo-RuClPPh(3)(mu,eta(6)-C(6)H(5)PPh(2))]-7,11-(mu-H)(2)-2,1-RuCB(10)H(8)R] (5, R = H; 6, R = 6-MeO; 7, R = 3-MeO) and [closo-2-(eta(6)-C(6)H(6))-10,11,12-[exo-RuCl(PPh(3))(2)]-10,11,12-(mu-H)(3)-2,1-RuCB(10)H(7)R(1)] (8a, R(1) = 6-MeO; 8b, R(1) = 3-MeO, inseparable mixture of isomers) along with trace amounts of 10-vertex mononuclear hypercloso/isocloso-type complexes [2,2-(PPh(3))(2)-2-H-3,9-(MeO)(2)-2,1-RuCB(8)H(7)] (9) and [2,5-(Ph(3)P)-2-Cl-2-H-3,9-(MeO)(2)-2,1-RuCB(8)H(6)] (10). Binuclear ruthenacarborane clusters of both series were characterized by a combination of analytical and multinuclear NMR spectroscopic data and by single-crystal X-ray diffraction studies of three selected complexes, 6-8. In solution, isomers 8a,b have been shown to undergo the isomerization process through the scrambling of the exo-[RuCl(PPh(3))(2)] fragment about two adjacent triangular cage boron faces B(7)B(11)B(12) and B(8)B(9)B(12).