New Heteronuclear Bridged Borylene Complexes That Were Derived from [{Cp*CoCl}₂] and Mono‐MetalCarbonyl Fragments

Abstract

The synthesis, structural characterization, and reactivity of new bridged borylene complexes are reported. The reaction of [{Cp*CoCl}2] with LiBH4·THF at -70 °C, followed by treatment with [M(CO)3(MeCN)3] (M=W, Mo, and Cr) under mild conditions, yielded heteronuclear triply bridged borylene complexes, [(μ3-BH)(Cp*Co)2(μ-CO)M(CO)5] (1-3; 1: M=W, 2: M=Mo, 3: M=Cr). During the syntheses of complexes 1-3, capped-octahedral cluster [(Cp*Co)2(μ-H)(BH)4{Co(CO)2}] (4) was also isolated in good yield. Complexes 1-3 are isoelectronic and isostructural to [(μ3-BH)(Cp*RuCO)2(μ-CO){Fe(CO)3}] (5) and [(μ3-BH)(Cp*RuCO)2(μ-H)(μ-CO){Mn(CO)3}] (6), with a trigonal-pyramidal geometry in which the μ3-BH ligand occupies the apical vertex. To test the reactivity of these borylene complexes towards bis-phosphine ligands, the room-temperature photolysis of complexes 1-3, 5, 6, and [{(μ3-BH)(Cp*Ru)Fe(CO)3}2(μ-CO)] (7) was carried out. Most of these complexes led to decomposition, although photolysis of complex 7 with [Ph2P(CH2)(n)PPh2] (n=1-3) yielded complexes 9-11, [3,4-(Ph2P(CH2)(n)PPh2)-closo-1,2,3,4-Ru2Fe2(BH)2] (9: n=1, 10: n=2, 11: n=3). Quantum-chemical calculations by using DFT methods were carried out on compounds 1-3 and 9-11 and showed reasonable agreement with the experimentally obtained structural parameters, that is, large HOMO-LUMO gaps, in accordance with the high stabilities of these complexes, and NMR chemical shifts that accurately reflected the experimentally observed resonances. All of the new compounds were characterized in solution by using mass spectrometry, IR spectroscopy, and (1)H, (13)C, and (11)B NMR spectroscopy and their structural types were unequivocally established by crystallographic analysis of complexes 1, 2, 4, 9, and 10.

First‐Row Transition‐Metal–Diborane and –Borylene Complexes

“…The core geometries of 2 and 4 are the same; the only difference is associated with the attachment of carbonyl ligands and bridging hydrogen atoms. The CoCo (2.46 Å), the CoB (1.97–2.00 Å), and the BH (1.12 Å) bond lengths compare well to those of previously reported examples 29. 37 According to the skeletal electron counts48 a tetrahedral geometry should contain six skeletal electron pairs, nevertheless 4 possesses seven.…”

“…A Co1Co2 bond length of 2.46 Å is in the range observed for a bond order of one. Similar situations have also been observed in [(μ 3 ‐BH)(Cp*Co) 2 (μ‐CO)M(CO) 5 ] (M=Cr, Mo and W) 29. Although the X‐ray analysis fails to locate the hydrogen atoms in 2 , except the BH terminal proton, 1 H NMR spectroscopy clearly verified their positions.…”

“…However, the nature of these interactions varies 2. 8c, 33–35 Earlier, we and others synthesized various trimetallic triply bridged borylene complexes, for example, [{(η 5 ‐C 5 Me 5 )Ru} 3 (μ‐H) 3 (μ 3 ‐BX)]36 (X=H, CN, OMe, OEt), [(CpCo) 3 (μ 3 ‐BPh)(μ 3 ‐PPh)],37 [(μ 3 ‐BX){[(η 5 ‐C 5 H 4 Me)Mn(CO) 2 ][Pd‐(PCy 3 )] 2 }]38, 40 (X= t Bu, Cl; Cy=cyclohexyl), [(μ 3 ‐BH)(Cp*RuCO) 2 (μ ‐ CO){Fe(CO) 3 }],39 [(μ 3 ‐BH)‐(Cp*RuCO) 2 (μ‐H)(μ‐CO){Mn(CO) 3 }],39 [(μ 3 ‐BH)(Cp*TaCO) 2 (μ ‐ CO){Fe(CO) 3 }],39 [(Cp*Co) 2 (μ 3 ‐BH)(μ‐CO)M(CO) 5 }]29 (M=Cr, Mo, and W), [(Cp*Fe(CO)(μ‐CO)M(PCy 3 )(μ‐Br)Pt(PCy 3 )Br(μ 3 ‐B)] (M=Pd, Pt; Cy=cyclohexyl),41 and [(Cp*RuCO) 3 (μ 3 ‐H)BH]42 (Table 1, see below). As an extension of our earlier work, we set out to synthesize new species containing other metal‐carbonyl fragments.…”

“…Studies on the recognition, classification, and insertion of borylene ligands have been carried out by the groups of Braunschweig, Aldridge, Sabo ‐ Etienne, and others 23–28. During the course of our studies on heterometallic triply bridged borylene complexes, we have recently isolated a series of triply bridged borylene complexes, [(μ 3 ‐BH)(Cp*Co) 2 (μ‐CO)M(CO) 5 ] (M=Cr, Mo, and W; Cp*=1,2,3,4,5‐pentamethylcyclopentadienyl), from the reaction of [M(CO) 3 (CH 3 CN) 3 ] (M=Cr, Mo and W) and an in situ‐generated cobaltaborane intermediate 29. As [M(CO) 5 ] − (M=Cr, Mo, W) is isolobal with [M(CO) 5 ] (M=Mn, Re),30 we turned our attention to the synthesis of analogous homo‐ and heterometallic bridged borylene compounds.…”

New Trinuclear Complexes of Group 6, 8, and 9 Metals with a Triply Bridging Borylene Ligand

“…Alternatively, the geometry of 3 and 4 can be viewed as a tetrahedral structure with three metals and one BH ligand. Compounds 3 and 4 are electronically and structurally analogous to [(Cp*Co) 2 (μ‐CO)(μ 3 ‐BH)M′(CO) 5 ], (M′=Cr, Mo, W) . The boron atom is situated approximately 1.402 Å (M′=Mo) and approximately 1.420 Å (M′=W) above the Rh1‐Rh2‐M′ plane in 3 and 4 , respectively.…”