Sundargopal Ghosh

Building upon previous studies on the synthesis of bis(sigma)borate and agostic complexes of ruthenium, the chemistry of nido-[(Cp Ru)2B3H9] (1) with other ligand systems was explored. In this regard, mild thermolysis of nido-1 with 2-mercaptobenzothiazole (2-mbzt), 2-mercaptobenzoxazole (2-mbzo) and 2-mercaptobenzimidazole (2-mbzi) ligands were performed which led to the isolation of bis(sigma)borate complexes [Cp RuBH3L] (2a-c) and β-agostic complexes [Cp RuBH2L2] (3a-c; 2a, 3a: L=C7H4NS2; 2b, 3b: L=C7H4NSO; 2c, 3c: L=C7H5N2S). Further, the chemistry of these novel complexes towards various diphosphine ligands was investigated. Room temperature treatment of 3a with [PPh2(CH2)nPPh2] (n=1-3) yielded [Cp Ru(PPh2(CH2)nPPh2)-BH2(L2)] (4a-c; 4a: n=1; 4b: n=2; 4c: n=3; L=C7H4NS2). Mild thermolysis of 2a with [PPh2(CH2)nPPh2] (n=1-3) led to the isolation of [Cp Ru(PPh2(CH2)nPPh2)(L)] (L=C7H4NS2 5a-c; 5a: n=1; 5b: n=2; 5c: n=3). Treatment of 4a with terminal alkynes causes a hydroboration reaction to generate vinylborane complexes [Cp Ru(R-C=CH2)BH(L2)] (6 and 7; 6: R=Ph; 7: R=COOCH3; L=C7H4NS2). Complexes 6 and 7 can also be viewed as η-alkene complexes of ruthenium that feature a dative bond to the ruthenium centre from the vinylinic double bond. In addition, DFT computations were performed to shed light on the bonding and electronic structures of the new compounds.

A combined experimental and quantum chemical study of Group 7 borane, trimetallic triply bridged borylene and boride complexes has been undertaken. Treatment of [{Cp∗CoCl}2] (Cp∗=1,2,3,4,5-pentamethylcyclopentadienyl) with LiBH4·thf at -78°C, followed by room-temperature reaction with three equivalents of [Mn2(CO)10] yielded a manganese hexahydridodiborate compound [{(OC)4Mn}(η6-B2H6){Mn(CO)3}2(μ-H)] (1) and a triply bridged borylene complex [(μ3-BH)(Cp∗Co)2(μ-CO)(μ-H)2MnH(CO)3] (2). In a similar fashion, [Re2(CO)10] generated [(μ3-BH)(Cp∗Co)2(μ-CO)(μ-H)2ReH(CO)3] (3) and [(μ3-BH)(Cp∗Co)2(μ-CO)2(μ-H)Co(CO)3] (4) in modest yields. In contrast, [Ru3(CO)12] under similar reaction conditions yielded a heterometallic semi-interstitial boride cluster [(Cp∗Co)(μ-H)3Ru3(CO)9B] (5). The solid-state X-ray structure of compound 1 shows a significantly shorter boron-boron bond length. The detailed spectroscopic data of 1 and the unusual structural and bonding features have been described. All the complexes have been characterized by using 1H, 11B, 13C NMR spectroscopy, mass spectrometry, and X-ray diffraction analysis. The DFT computations were used to shed light on the bonding and electronic structures of these new compounds. The study reveals a dominant B-H-Mn, a weak B-B-Mn interaction, and an enhanced B-B bonding in 1.