J Clust Sci DOI 10.1007/s10876-014-0736-y
ORIGINAL PAPER
Metal Clusters. As They Were Born in Siberia
Vladimir Fedorov
Received: 1 April 2014 Ó Springer Science+Business Media New York 2014
Abstract Key results of the researches in the field of cluster chemistry executed by scientists of Nikolaev Institute of Inorganic Chemistry (NIIC) of the Siberian Branch of Russian Academy of Sciences are presented. Structure and properties of some cluster compounds of niobium, tantalum, molybdenum, tungsten and rhenium for the first time synthesized in NIIC are briefly discussed. Some original results which are conceptually important in chemistry of metal cluster complexes are noted.
Keywords Metal clusters Niobium Molybdenum Rhenium Chemical modification Condensation of cluster fragments
Introduction
The coordination chemistry of the transition metals always was one of the central directions of chemical science. The main features of the coordination compounds were formulated more than a century ago by Werner [1] and during long time this classical work regularly served as the theoretical base of coordination chemistry. However in the mid-sixties of the last century there came peculiar ‘‘crisis’’ of the one-center Werner’s theory. Experimenters even more often found polynuclear compounds with very short contacts between metal atoms. These facts demanded an exit for a framework of one-center theory. In 1964 F. A. Cotton [2, 3] introduced the term ‘‘metal atom cluster compounds’’ which defined groups of polynuclear metal complexes with direct metal–metal bonds. From this point there is a keen interest in
V. Fedorov (&) Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Sciences, 3, Acad. Lavrentiev Prospect, Novosibirsk State University, 2, Pirogova St, Novosibirsk 630090, Russian Federation e-mail: [email protected] 123 V. Fedorov metal cluster complexes which keeps so far steady. Today we celebrate 50-year anniversary of the cluster chemistry. In anniversary days there is a natural desire to look back and to sum up some results. In this article we present the studies on the transition metals cluster complexes performed in Nikolaev Institute of Inorganic Chemistry of the Siberian Branch of the Russian Academy of Sciences which from the very beginning actively joined in development of the new scientific direction and made a significant contribution to cluster chemistry. In the present short contribution there is no possibility to comprehensively present all researches executed by scientists of NIIC for last years in the field of cluster chemistry. Therefore, the author had to focus only on some specific aspects of chalcogenide and chalcohalogenide clusters that are closer to the author as to the researcher. We will consider here only some key cluster systems of this type for the first time synthesized at the Institute that are most characteristic representatives for 4d and 5d transition metals (Fig. 1).
First Studies: From Where Everything Came
Early studies of metal cluster compounds in NIIC were begun in Prof. A. Opalov- sky’s laboratory at the beginning-sixties of last century: P. Samoylov, the student of Novosibirsk State University (NSU), studied the chemistry of molybdenum ‘‘dibromide’’. As now we know, it is one of the most typical hexanuclear octahedral cluster complex Mo6Br12 (it should note in brackets that at that time similar compounds according to Brosset’s proposal called a staphyle´ [4]). In the monograph ‘‘Molybdenum halogenides’’ published in 1972 the certain head is devoted to the staphyle´ complexes [5]. Studying Mo/Se and Mo/Te systems it was revealed that the lowest molybdenum selenide and telluride had analytical composition of Mo3Q4 (Q = Se, Te) [6–8]. Later, using single crystal X-ray diffraction it was shown that crystal structures of these binary chalcogenides contain an octahedral metal cluster and, therefore, they should be described as Mo6Q8. This structural type can be considered as an ancestor of the wide group of related ternary compounds MMo6Q8 (M = Pb, Sn and other metals; Q = S, Se), which received in literature the name of ‘‘Chevrel phases’’ in honor of the French PhD student R. Chevrel who synthesized the first compounds with tin and lead [9]. These ternary chalcogenides were investigated very intensively in connection with their superconducting properties: for that period of time some compounds of this type showed critical temperatures about 15 K and record-breaking critical fields (about 60 Teslas) [10]. Today the term ‘‘Chevrel phases’’ was included in educational and encyclopedic editions, like Zintl phases, Laves phases, etc. In the same time, studying reactions of the lowest molybdenum halogenides with chalcogens (executed by V. Fedorov under strong influence of the ideas of Prof. S. Batsanov who was keen on synthesis of compounds with the mixed ligands, in particular chalcohalogenides) led to opening of three-nuclear triangle complexes Mo3Q7X4 (Q = S, Se; X = Cl, Br, I) [11, 12] (Fig. 1a). Probably, these polymeric compounds having low reactivity and being insoluble in usual solvents would 123 Metal Clusters
Fig. 1 Examples of metal cluster complexes of different nuclearity discovered in NIIC remain unnoticed if ways of their transformation in molecular soluble forms were not found. Pioneer works devoted to transformation of similar polymeric compounds into soluble complexes were carried out by V. Fedin [13, 14] (Fig. 2). The idea consisted in a breaking of bridged bonds in compounds with polymeric structures by means of reactions of solids with strong nucleophilic ligands. In such reactions the architecture and composition of the cluster core {Mo3S7} were remained; examples of these reactions are given below: [{Mo3S7}Cl2Cl4/2]? ? 2 PPh3 ? [{Mo3S7}Cl4(PPh3)2] [{Mo3S7}Cl2Cl4/2]? ? 3KS2P(OEt)2 ? [{Mo3S7}(S2P(OEt)2)3]Cl The range of the depolymerization reactions was significantly expanded by M. Sokolov, O. Gerasko and A. Gushchin by means of the application of mechano- chemical activation of the processes [15]. Some of such reactions are given on Fig. 3. Later such processes of a depolymerization of metal cluster polymers received the name as ‘‘excision reactions’’ of cluster core. These approaches opened a way to wide-ranging studies of triangular chalcogenide clusters by solution chemistry. These complexes were ancestors of the whole family of triangular cluster complexes of molybdenum and the tungsten containing cluster cores {M3(l3-S)(l2-S2)3} and {M3(l3-S)(l2-S)3}. So far in the world literature several hundred articles devoted to structure and chemistry of similar complexes are published [16–21]. It is necessary to mention also a series of works on binuclear chalcogenide cluster complexes of niobium, molybdenum and tungsten for which original methods of synthesis were offered and their chemical properties were carefully studied (V. Fedin, M. Sokolov, O. Gerasko [22, 23]). As an example, the developed scheme for preparation of volatile binuclear niobium complexes Nb2S4L4 with organic ligands
123 V. Fedorov
potassium diethyldithiocarbamate MeCN, reflux, 1 h
Fig. 2 Excision of cluster core {Mo3S7} from polymeric structure of triangle cluster complex [{Mo3S7}Cl2Cl4/2]? by reaction with diethyldithiocarbamate
Fig. 3 Scheme of preparation of soluble cluster complexes by mechanochemical activation of reactions of solids
L is presented below. It consists of a chain of the consecutive transformations: (i) high-temperature reaction of elements leading to formation of a cluster polymer Nb2S4Br4; (ii) transformation of the polymeric compound Nb2S4Br4 to an anionic complex in molten KSCN; (iii) obtaining an aquacomplex by an exchange of terminal ligands SCN- in the course of acid hydrolysis and, at last, (iv) replacement of water molecules by organic ligands with isolation of target compounds Nb2S4L4 - - - (L = CH3COCHCOCH3 ,CF3COCHCOCH3 ,CF3COCHCOC(CH3)3 ,CF3COC - - - HCOCFC5F10 ,CF3COCHCOC6H5 ,S2CN(C2H5)2 ):