Pocket Semiquinonate Complexes of Cobalt(II), Copper(II), and Zinc(II) Prepared with the Hydrotris(cumenylmethylpyrazolyl)borate Ligand Michael Ruf, Bruce C. Noll, Markus D. Groner, Gordon T. Yee, and Cortlandt G. Pierpont* Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309 ReceiVed February 28, 1997 X 3,5-Di-tert-butyl-1,2-semiquinonate (3,5-DBSQ) complexes of Co(II), Cu(II), and Zn(II) have been prepared that contain the hydrotris(cumenylmethyl-pyrazolyl)borate (Tp Cum,Me ) coligand. Tp Cum,Me Zn(3,5-DBSQ) and Tp Cum,Me - Cu(3,5-DBSQ) were prepared by treating the parent hydroxide, Tp Cum,Me M(OH), M ) Cu and Zn, with 3,5-di- tert-butylcatechol. Tp Cum,Me Co(3,5-DBSQ) was prepared by a reaction between (Tp Cum,Me ) 2 Co and 3,5-DBCat. The identity of (Tp Cum,Me ) 2 Co in this reaction was confirmed by a structure determination [(Tp Cum,Me ) 2 Co: orthorhombic, Pbcn, a ) 17.7189(4) Å, b ) 17.4806(3) Å, c ) 25.7123(6) Å, V ) 7964.1(3) Å 3 , Z ) 4, R(F) ) 0.054]. Intersecting cumenyl substituents of the pyrazolylborate ligand encapsulate the Co(II) ion. Structural characterization on all three members of the Tp Cum,Me M(3,5-DBSQ) series has been carried out. The complexes of Co(II) and Zn(II) are isomorphous and isostructural [Tp Cum,Me Co(3,5-DBSQ): triclinic, P1h, a ) 14.4631(2) Å, b ) 18.5438(3) Å, c ) 21.6142(2) Å, R) 79.8430(10)°,  ) 90.0900(10)°, γ ) 84.9900(10)°, V ) 5683.45(13) Å 3 , Z ) 4, R(F) ) 0.072; Tp Cum,Me Zn(3,5-DBSQ), triclinic, P1h, a ) 14.261(3) Å, b ) 18.760(7) Å, c ) 21.710- (4) Å, R) 80.049(12)°,  ) 89.853(8)°, γ ) 85.542(12)°, V ) 5703(3) Å 3 , Z ) 4, R(F) ) 0.064]. Tp Cum,Me - Cu(3,5-DBSQ) [monoclinic, P2 1 /c, a ) 19.3081(3) Å, b ) 13.0291(2) Å, c ) 21.4783(4) Å,  ) 102.8420(10)°, V ) 5268.1(2) Å 3 , Z ) 4, R(F) ) 0.071] has a distorted square pyramidal structure, the complexes of Zn and Co have structures that are closer to a trigonal bipyramid. Parent catecholate complexes of all three metals are unusually stable in air but undergo slow oxidation in solution to give the semiquinonate products characterized structurally. Copper(II) and SQ spins of Tp Cum,Me Cu(3,5-DBSQ) are located in orthogonal orbitals, and the complex has a S ) 1 spin state. The charge distribution in Tp Cum,Me Co(3,5-DBSQ) is Co(II)-SQ, rather than the more common Co(III)-Cat, due to surprisingly weak donation by the Tp Cum,Me nitrogens. Introduction Much of the coordination chemistry of catecholate and radical semiquinonate ligands has been concerned with electron transfer and magnetism. 1 Unless stabilized by strong π donation to an electron deficient metal ion, catecholate (Cat) ligands are strong reducing agents. Aerobic oxidation of coordinated catechol ligands may lead to oxidative cleavage of the aromatic ring by an inner-sphere mechanism that may involve a peroxysemi- quinone intermediate. 2 Outer-sphere oxidation results in forma- tion of a coordinated semiquinonate (SQ) ligand with partial dioxygen reduction to superoxide. 3 Aerobic oxidation of a parent catecholate complex is a convenient synthetic route to complexes containing SQ radical anions. Magnetic measure- ments have been used to study metal-SQ exchange for compounds containing paramagnetic metal ions and SQ-SQ exchange for compounds containing coupled radical ligands. 4 Valence tautomerism has been studied for complexes of Co and Mn as a special example of inner-sphere metal-ligand electron transfer. 5 These investigations have involved equilibria between redox isomers differing in the distribution of charge between metal and quinone ligand for the M II (N-N)(SQ) 2 ,M ) Co and Mn, series. Ruf and Vahrenkamp recently described the chemistry of Tp Cum,Me Zn(OH). 6 The hydroxo ligand of this complex is a strong base contained within a protective pocket created by the cumenyl substituents of the tris(pyrazolyl)borate ligand. This complex has been used to prepare Tp Cum,Me Zn(3,5-DBSQ) by reaction with H 2 (3,5-DBCat). Similar synthetic procedures have been used to form Tp Cum,Me Cu(OH) and Tp Cum,Me Cu(3,5-DBSQ). The addition of K(Tp Cum,Me ) to Co 2+ has been found to give (Tp Cum,Me ) 2 Co, and this product has been used as a precursor to Tp Cum,Me Co(3,5-DBSQ). The quinone ligand of each of these compounds is encapsulated by the substituents of the Tp Cum,Me ligand contributing to unique electronic and magnetic properties. Experimental Section Materials. 3,5-Di-tert-butylcatechol, H2(3,5-DBCat), was purchased from Aldrich. K(Tp Cum,Me ) and Tp Cum,Me Zn(OH) were prepared by procedures described by Ruf and Vahrenkamp. 6 Tp Cum,Me Zn(3,5-DBSQ). Tp Cum,Me Zn(OH) (200 mg, 0.29 mmol) was dissolved in 50 mL of a 1:1 dichloromethane/methanol solution. H 2(3,5-DBCat) (64 mg, 0.29 mmol) was added, and the mixture was stirred for 30 min. The solution was initially colorless. After exposure to air for the period of 12 h the solution turned green, and dark green crystals formed. The crystals were separated from the solution by filtration and dried in Vacuo to give 211 mg of Tp Cum,Me Zn(3,5-DBSQ) in 82% yield. Tp Cum,Me Cu(OH). K(Tp Cum,Me ) (1.00 g, 1.54 mmol) was dissolved in 100 mL of dichloromethane, and a methanol (50 mL) solution of Cu(ClO 4)2‚6H2O (571 mg, 1.54 mmol) was added. After stirring of the green reaction mixture for 15 min, KOH (86 mg, 1.54 mmol) was X Abstract published in AdVance ACS Abstracts, September 15, 1997. (1) Pierpont, C. G.; Lange, C. W. Prog. Inorg. Chem. 1994, 41, 331. (2) (a) Speier, G.; Tyekla´r, Z.; Szabo´, II, L; To´th, P.; Pierpont, C. G.; Hendrickson, D. N. In The ActiVation of Dioxygen and Homogeneous Catalytic Oxidation; Barton, D. H. R., Martell, A. E., Sawyer, D. T., Eds.; Plenum: New York, 1993; p 423. (b) Barbaro, P.; Bianchini, C.; Linn, K.; Mealli, C.; Meli, A.; Vizza, F.; Laschi, F.; Zanello, P. Inorg. Chim. Acta 1992, 198-200, 31. (3) Hubbard, C. D.; Gerhard, A.; van Eldik, R. Inorg. Chem. 1991, 30, 5023. (4) (a) Attia, A. S.; Conklin, B. J.; Lange, C. W.; Pierpont, C. G. Inorg. Chem. 1996, 35, 1033. (b) Lange, C. W.; Conklin, B. J.; Pierpont, C. G. Inorg. Chem. 1994, 33, 1276. (5) (a) Jung, O.-S.; Jo, D. H.; Lee, Y.-A; Conklin, B. J.; Pierpont, C. G. Inorg. Chem. 1997, 36, 19. (b) Attia, A. S.; Pierpont, C. G. Inorg. Chem. 1995, 34, 1172. (6) Ruf, M.; Vahrenkamp, H. Inorg. Chem. 1996, 35, 6571. 4860 Inorg. Chem. 1997, 36, 4860-4865 S0020-1669(97)00244-9 CCC: $14.00 © 1997 American Chemical Society