Clusters) CHEMISTRY. Paper 11: Inorganic Chemistry III ( Metal π-complexes and Metal Clusters ) Module 26: Metallocarboranes and their preparation

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1 Subject Chemistry Paper No and Title Module No and Title Module Tag 11: Inorganic Chemistry III (Metal π-complexes and Metal Clusters) CHE_P11_M26

2 TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 3. General methods of preparation of metallocarboranes 5. Summary

3 1. Learning Outcomes After studying this module, you shall be able to Know about preparative methods of metallocarboranes. 2. Introduction The chemistry of metallocarboranes is a vast and rewarding field of study. Metallocarboranes are much less reactive (more stable) than the corresponding metallocenes and these tend to stabilize higher oxidation state of later transition metals, e.g. [Cu II (1,2-C 2B 9H 11) 2] 2- and [Cu III (1,2- C 2B 9H 11) 2] - are known whereas cuprocene [Cu II (η 5 -C 5H 5) 2] is not. Fe(III) and Ni(IV) carborane derivatives are extremely stable. Metallocarboranes tend to stabilize lower oxidation states of early transition elements and complexes are well established for Ti(II),Zr(II),Hf(II),V(II),Cr(II) and Mn(II).These do not react with H 2, N 2,CO or PPh 3 as do cyclopentadienyl derivatives of these elements. 3. General methods of preparation The methods of preparation of metallocarboranes are enormous. The six major synthetic methods of metallocarboranes are (i) coordination using nido-carborane anions as ligands (ii) polyhedral expansion (iii) polyhedral contraction (iv) polyhedral subrogation (v) thermal metal transfer and (vi) direct oxidative insertion of the metal centre. All except the last were developed by M.F. Hawthorne and his group in the period and have since then been extensively exploited by several groups. (i) Coordination using nido-carborane anions as ligands (1965)- The reaction of dicarbollide anion C 2B 9H with FeCl 2 in tetrahydrofuran (thf) with rigorous exclusion of moisture and air gives the pink, diamagnetic bis-sandwich-type complex of Fe(II) (Fig.1) which can be reversibly oxidized to the corresponding red Fe(III) complex

4 Fig.1 Pink sandwich-type complex of Fe(II) If the reaction is carried out in the presence of NaC 5H 5, the purple mixed sandwich complex is obtained (Fig.2) Fig.2 Purple mixed sandwich complex of Fe(III) The reaction is general and has been applied to many transition metals Cr,Mo, W; Mn,Re, Ni, Pd, Cu,Ag, Au and Be. The use of metal carbonyls supplies the capping unit, e.g. (ii) Polyhedral expansion (1970)- The 2-electron reduction of a closo-carborane with a strong reducing agent such as sodium naphthalide in thf followed by reaction

5 with a transition-metal reagent is quite general for closo-carboranes. It involves the 2- electron reductive opening of an n-vertex closo-cluster followed by metal insertion to give an (n+1)-vertex closo-cluster. The insertion of a second metal centre into metallocarboranes leads to the formation of a bimetallic complex having 10-vertex cluster as(fig.3). Fig.3 Bimetallic complex having 10-vertex cluster The reaction of cobalt chloride with icosahedral 1,2-C 2B 10H 12 gives supraicosahedral metallacarboranes with 13- and 14-vertex polyhedral structures [Scheme 1(b-e)].

6 Scheme1.(a)icosahedral 1,2-C 2B 10H 12 (b-e)facile isomerism of the 13-vertex monometallodicarbaboranes (iii) Polyhedral contraction (1972)- This involves the removal of one BH group from a closo-metallocarborane by nucleophilic base degradation followed by oxidative closure of the resulting nido-metallocarborane complex to a closo-species with one vertex less than the original,(fig.4)e.g.: Fig.4 Formation of closo-complex by base catalyzed reaction and H 2O 2

7 This method is less used for the preparation of metallacarboranes compared to polyhedral expansion since some metallacarboranes degrade completely under these conditions. (iv) Polyhedral subrogation (1973). The replacement of a BH vertex by a metal vertex without changing the number of vertices in the cluster is termed polyhedral subrogation. The degradative removal of the BH unit is followed by reaction with a transition metal ion rather than with an oxidizing agent, e.g.: The method is of potential use in preparing mixed metal clusters, e.g. (Co + Ni or (Co + Fe), and can be extended to prepare more complicated cluster arrays as shown in Fig.5, the subrogated B atom being indicated as a broken circle. Fig.5 Formation of mixed metal clusters (v) Thermal metal transfer (1974)- This method is less general and often less specific than the coordination of nido-anions of polyhedral expansion.it involves the pyrolysis of pre-existing metallacarboranes and consequent cluster expansion or disproportionation similar to that of the closo-carboranes yielding mixtures of products. Alternatively, the direct metal insertion by gas-phase reactions at elevated temperatures can be shown as (scheme 2). The reaction with [Co(η 5 -C 5H 5)(CO) 2] also gives the 7-vertex closobimetallocarborane which can be considered as a rare example of a triple-decker sandwich compound. Another isomer (Fig.6) can be made by base degradation of [{Co(η 5 -C 5H 5)}(C 2B 4H 6)] followed by deprotonation and subrogation with a second {Co(η 5 -C 5H 5)} unit having the central planar formal C 2B 3H 5 4- unit isoelectronic with C 5H 5-.

8 Scheme 2. Direct metal insertion by gas-phase reactions

9 Fig.6 (a) 7-vertex closo-bimetallocarborane (b) Isomer of closo-bimetallocarborane (vi) Direct oxidative insertion of a metal centre (F.G.A.Stone et al., 1972)- The nucleophilic zero-valent derivatives of Ni, Pd, and Pt insert directly into closo-carborane clusters in a concerted process which involves a net transfer of electrons from the metal to the cage. A typical reaction is Pt-(PEt 3 ) 3 + 2,3-Me 2-2,3-C 2 B 9 H 9-30 o C [1-{Pt-(PEt 3 ) 2 }-2,4-(MeC) 2 B 9 H 9 ] + PEt 3 Petrol 5. Summary The metallocarboranes can be prepared by six major synthetic methods Coordination using nido-carborane anions as ligands Polyhedral expansion Polyhedral contraction Polyhedral subrogation Thermal metal transfer and Direct oxidative insertion of the metal centre.