The synthesis and structural characterization of the hitherto unknown parent Co(bz) (bz=benzene) complex and several of its derivatives are described. Their synthesis starts either from a CoCO salt, or directly from Co (CO) and a Ag salt. Stability and solubility of these complexes was achieved by using the weakly coordinating anions (WCAs) [Al(OR ) ] and [F{Al(OR ) } ] {R =C(CF ) } and the solvent ortho-difluorobenzene (o-DFB). The magnetic properties of Co(bz) were measured and compared in the condensed and gas phases. The weakly bound Co(o-dfb) salts are of particular interest for the preparation of further Co salts, for example, the structurally characterized low-coordinate 12 valence electron Co(P Bu ) and Co(NHC) salts.
The facile synthesis of a pentacarbonyl cobalt(I) salt without the need for a superacid as solvent is presented. This salt, [Co(CO) ] [Al(OR ) ] {R =C(CF ) }, readily accessible on a multigram scale, undergoes substitution reactions with arenes yielding the hitherto unknown class of two-legged cobalt piano-stool complexes [(arene)Co(CO) ] with four different arene ligands. Such a substitution chemistry would have been impossible in superacid solution, as the arenes used would have been oxidized and/or protonated. Thus, the general approach described herein may have a wide synthetic use. Additionally, the thermochemistry of the piano-stool complexes is shown to be not easy to describe computationally and most of the established DFT methods overestimate the reaction energies. Only CCSD(T) calculations close to the basis set limit gave energies fully agreeing with the experiment.
The synthesis and structural characterization of the hitherto unknown parent Co(bz) 2 + (bz = benzene) complex and several of its derivatives are described. Their synthesis starts either from aC oCO 5 + salt, or directly from Co 2 (CO) 8 and aAg + salt. Stability and solubility of these complexes was achieved by using the weakly coordinating anions (WCAs) [Al(OR F ) 4 ] À and [F{Al(OR F ) 3 } 2 ] À {R F = C(CF 3 ) 3 }a nd the solvent ortho-difluorobenzene (o-DFB). The magnetic properties of Co(bz) 2 + were measured and compared in the condensed and gas phases.T he weakly bound Co(o-dfb) 2 + salts are of particular interest for the preparation of further Co I salts, for example,t he structurally characterized low-coordinate 12 valence electron Co(P t Bu 3 ) 2 + and Co(NHC) 2 + salts.After the discovery of bis(benzene)chromium as the first sandwich compound bearing two neutral arene ligands in 1955, [1] it did not take long until the first cobalt analogue Co(hmb) 2 + [PF 6 ] À (hmb = hexamethylbenzene) was reported as ap aramagnetic 20 valence electron (VE) complex with at riplet spin state configuration, [2] of which the crystal structure remains the only reported example. [3] Subsequent attempts to isolate Co(arene) 2 + complexes with arenes other than the very electron rich hmb (e.g.,w ith mesitylene, [2] toluene, [4] bz [4,5] )f ailed. Va rious Co(hmb) 2 + salts were the subject of further studies [6,7] and served for the synthesis of novel Co I compounds,for example,with dienes. [8,9] However, only one hmb ligand could be displaced and the fragment (hmb)Co + was always incorporated.By contrast, advanced mass spectrometric (MS) methods [10] have permitted the gas phase preparation and study of the parent cations M(bz) 2 + (M = transition metal). [11,12] The large majority of those cations remain unknown as stable salts in condensed phases,e .g., of M = Co,R ha nd Ir.A mongst those characterized is the gaseous Co(bz) 2 + cation, [12][13][14] reported also with IR data, [12] but currently without magnetic measurements.F rom ag eneral perspective,i tw ould be very interesting to compare the spectroscopic and magnetic data of gaseous Co(bz) 2 + and as table Co(bz) 2 + [X] À salt. Thus,i t represented an interesting fundamental challenge to work on both issues.From as ynthetic perspective,h omoleptic metal arene cations [M(arene) n ] + (n = 1-3) have received significant interest due to their possible applications in olefin oligomerization (M = Ga), [15] arene functionalizations and as building blocks for organometallic pharmaceuticals (M = Re,Tc) [16] as well as the unprecedented arene-to-cyclopentadienyl ring contraction (M = Tc). [17] Furthermore,u nivalent M(arene) n + complexes with more electron-deficient fluoroarenes,l ike fluorobenzene (fb) or o-dfb,c an be considered as sources of "naked" metal ions M + .T hus,for M = Ga I ,In I as eries of P-,
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