Amidinato– and Guanidinato–Cobalt(I) Complexes: Characterization of Exceptionally Short Co–Co Interactions

Abstract

Low-coordinate, carbonyl-free first row transition metal(I) complexes are relatively rare but are finding increasing use in the activation of small molecules, as enzyme mimics, and so forth. These complexes are generally very reactive species that are stabilized by a variety of sterically bulky, mono-, di-, tri-, and higher dentate ligands. Perhaps the most versatile of these are the b-diketiminates (e.g., [{ArNC(Me)}2CH] (nacnac ; Ar= 2,6-diisopropylphenyl)), which have been utilized in the preparation of a range of Group 5–12 first row transition metal(I) complexes that have shown fascinating chemistry. In recent years, we have employed the bulky amidinate and guanidinate ligands ([(ArN)2CR] (R= tBu; piso ), N(C6H11)2 (giso ), or NiPr2 (priso )) for the stabilization of a variety of Group 2, 13, 14, and 15 metal(I) complexes, and planar four-coordinate lanthanide(II) complexes. These studies have highlighted close analogies (but also differences) between the stabilizing and ligating properties of the bulky amidinates and guanidinates, and the properties of b-diketiminates. With these characteristics in mind, we extended the coordination chemistry of the bulky ligand piso to the preparation of the first amidinato–iron(I) complex, [(k-N,N’-piso)Fe(h-toluene)] (cf. [(k-N,N’-nacnac)Fe(h-benzene)]), which was shown to weakly activate dinitrogen to give [{(k-N-,h-Ar-piso)Fe(m-N)}2] (cf. [{(k N,N’-nacnac)Fe(m-N)}2] ), with an accompanying change in the coordination mode of the piso ligand. Subsequent reports from another research group detailed unprecedented amidinato–chromium(I) complexes, which included the diamagnetic, amidinate bridged species, [{Cr[m-N(Ar’)C(R)N(Ar’)]}2] (R=H or Me, Ar’=Ar or 2,6-Et2C6H3), that contain very short Cr–Cr quintuple bonds (ca. 1.74 ). These results motivated us to extend the coordination chemistry of bulky amidinate and guanidinate ligands toward other first row transition metal(I) centers. We were particularly interested in preparing analogues of the bdiketiminate-stabilized cobalt(I) system [(k-N,N’-nacnac)Co(h-toluene)] (1), which, like other cobalt(I) complexes, has been shown to activate an assortment of small molecules. In addition, we believed that the previously demonstrated coordinative flexibility of our ligands relative to that of b-diketiminates could yield varying complex types, depending on the reaction conditions employed. Herein, we report the first amidinato– and guanidinato– cobalt(I) complexes, two dimeric examples of which exhibit the shortest Co–Co interactions reported to date. Preliminary further reactivity studies of these complexes are also reported. The paramagnetic cobalt(II) precursor complexes 2a–c (Scheme 1), were readily prepared in good yields by saltmethathesis reactions between CoX2 (X=Br or I) and the potassium salt of the appropriate ligand. The structural characterization of one complex, [{(priso)CoI}2], is the first to be reported for an amidinato– or guanidinato–cobalt(II) halide complex, and shows the complex to be dimeric with distorted tetrahedral cobalt centers. Upon reduction of 2a–c with potassium (or magnesium) in toluene, the amidinato– and guanidinato–cobalt(I) complexes 3a–c were obtained as crystalline solids in high yields. It is noteworthy that no nitrogen-coordinated complexes were obtained when the reductions were carried out under a dinitrogen atmosphere, as was the case with the reduction in toluene that gave 1. Reduction of 2a or 2b with potassium in cyclohexane under a dinitrogen atmosphere afforded the dimeric cobalt(I) complexes 4a and 4b as extremely air-sensitive solids in good yields. We have seen no evidence so far for the conversion of [*] Prof. C. Jones, Dr. C. Schulten, Dr. R. P. Rose, Dr. A. Stasch, W. D. Woodul, Prof. K. S. Murray, Dr. B. Moubaraki School of Chemistry, Monash University PO Box 23, VIC, 3800 (Australia) Fax: (+61)3-9905-4597 E-mail: cameron.jones@sci.monash.edu.au