Ta(CNDipp)₆: An Isocyanide Analogue of Hexacarbonyltantalum(0)

Abstract

Hexakis(2,6-diisopropylphenylisocyanide)tantalum is the first isocyanide analogue of the highly unstable Ta(CO) 6 and represents the only well-defined zerovalent tantalum complex to be prepared by conventional laboratory methods. Tw op rior examples of homoleptic Ta 0 complexes are known, Ta(benzene) 2 and Ta(dmpe) 3 ,d mpe = 1,2-bis(dimethylphosphano)ethane,but these have only been accessed via ligand cocondensation with tantalum vapor in as ophisticated metalatom reactor.C onsistent with its 17-electron nature,T a-(CNDipp) 6 undergoes facile one-electron oxidation, reduction, or disproportionation reactions.I nt his sense,i tq ualitatively resembles V(CO) 6 ,t he only paramagnetic homoleptic metal carbonyl isolable under ambient conditions.Incontrast to vanadium and niobium, [1] isolable homoleptic complexes of zerovalent tantalum are quite rare,w ith only two examples presently known:T a(benzene) 2[2] and Ta-(dmpe) 3 , [3] dmpe = 1,2-bis(dimethylphosphano)ethane.A lso, these species are very poorly accessible because they are only available via tantalum vapor synthesis methods,which require specialized metal-atom reactors. [4] Although Ta(bipy) 3 ,bipy = a,a'-bipyridine, [5] and Ta(iPr 2 -dad) 3 , iPr 2 -dad = 1,4-diisopropyl-1,4-diazabuta-1,3-diene, [6] were originally suggested to be zerovalent tantalum complexes,later experimental and computational studies indicate that both compounds contain high-valent Ta V . [7] Perhaps the most interesting comparisons of homoleptic zerovalent complexes of Group 5m etals are for the 17electron hexacarbonyls of these species:V (CO) 6 is wellknown as the only paramagnetic unsubstituted metal carbonyl isolable under ambient conditions, [8] while Ta(CO) 6 [9] and Nb(CO) 6[10] have only been obtained at low temperatures as matrix isolated species and may be unstable above 50 K. Barybinss ynthesis of the 17-electron complex V(CNXyl) 6 , Xyl = 2,6-dimethylphenyl, which was far more thermally stable (m.p.1 59-160 8 8Cd ecomp.) [1i] than V(CO) 6 (decomp. 60-70 8 8C), [8] suggested that Ta(CNXyl) 6 and the niobium analog might be isolable.H owever,d espite success in the synthesis and isolation of [Ta(CNXyl) 7 ] + ,[Ta(CNXyl) 6 ] À ,and related complexes, [11, 12] attempts to isolate the corresponding elusive neutral homoleptic tantalum and niobium complexes uniformly failed. [13] Many years ago Mann, Gray,a nd Hammond reported on the zerovalent complex W(CNDipp) 6 , Dipp = 2,6-diisopropylphenyl, [14] and it was of interest to determine whether this more sterically encumbered isocyanide could stabilize the corresponding neutral tantalum complex. Initial results were promising and provided astructural characterization of the unique ditantalum salt [Ta-(CNDipp) 7 ][Ta(CNDipp) 6 ], [15] atype of compound previously unknown for homoleptic metal carbonyls and isocyanides, [16] and of significance as aformal disproportionation product of the desired Ta(CNDipp) 6 complex. Unfortunately,t his salt could not be obtained as ap ure bulk solid and the neutral species was not isolated, so ...