Stepwise Cluster Assembly Using VO₂(acac) as a Precursor:  cis-[VO(OCH(CH₃)₂)(acac)₂], [V₂O₂(μ-OCH₃)₂(acac)₂(OCH₃)₂], [V₃O₃{μ,μ-(OCH₂)₃CCH₃}₂(acac)₂(OC₂H₅)], and [V₄O₄(μ-O)₂(μ-OCH₃)₂(μ₃-OCH₃)₂(acac)₂(OCH₃)₂]·2CH₃CN¹

Abstract

The studies of an underexplored synthetic reagent, VO(2)(acac) (Hacac = acetylacetone) and semirational strategies for the formation of a complete series of simple vanadium(V) alkoxide clusters in alcohol-containing solvents. The neutral mono-, di-, tri-, and tetranuclear oxovanadium(V) complexes [V(2)O(2)(&mgr;-OCH(3))(2)(acac)(2)(OCH(3))(2)] (1), [V(4)O(4)(&mgr;-O)(2)(&mgr;-OCH(3))(2)(&mgr;(3)-OCH(3))(2)(acac)(2)(OCH(3))(2)].2CH(3)CN (2), [V(4)O(4)(&mgr;-O)(2)(&mgr;-OCH(3))(2)(&mgr;(3)-OCH(3))(2)(acac)(2)(OCH(3))(2)] (3), [V(3)O(3){&mgr;,&mgr;-(OCH(2))(3)CCH(3)}(2)(acac)(2)(OR)] (R = CH(3) (4), C(2)H(5) (5)), and cis-[VO(OCH(CH(3))(2))(acac)(2)] (6) with alkoxide and acac(-) ligands were obtained by reaction of VO(2)(acac) with a monoalcohol and/or a tridentate alcohol. The structures of complexes 1-3, 5, and 6 were determined by X-ray diffraction methods. Complex 1 crystallized in the monoclinic system, P2(1)/n, with a = 7.8668(5) Å, b = 15.1037(9) Å, c = 8.5879(5) Å, beta = 106.150(1) degrees, V = 980.1(1) Å(3), Z = 2, and R (wR2) = 0.040 (0.121). Complex 2 crystallized in the monoclinic system, P2(1)/n, with a = 8.531(2) Å, b = 14.703(3) Å, c = 12.574(2) Å, beta = 95.95(2) degrees, V = 1568.7(5) Å(3), Z = 2, and R (wR2) = 0.052 (0.127). Complex 3 crystallized in the triclinic system, P&onemacr;, with a = 8.5100(8) Å, b = 8.9714(8) Å, c = 10.3708(10) Å, alpha = 110.761(1) degrees, beta = 103.104(1) degrees, gamma = 100.155(1) degrees, V = 691.85(11) Å(3), Z = 1, and R (wR2) = 0.040 (0.105). Complex 5 crystallized in the monoclinic system, P2(1)/n, with a = 14.019(2) Å, b = 11.171(2) Å, c = 19.447(3) Å, beta = 109.18(1) degrees, V = 2876.5(8) Å(3), Z = 4, and R (wR2) = 0.062 (0.157). Complex 6 crystallized in the monoclinic system, P2(1)/n, with a = 15.0023(8) Å, b = 8.1368(1) Å, c = 26.5598(2) Å, beta = 95.744(1) degrees, V = 3225.89(8) Å(3), Z = 8, and R (wR2) = 0.060 (0.154). Complex 1 is a discrete, centrosymmetric dimer in which two vanadium atoms are bridged by two methoxide ligands. Compound 2 contains a V(4)O(4) eight-membered ring with both &mgr;-oxo and &mgr;-alkoxo bridging ligands; the ring is capped above and below by two triply bridging methoxo ligands. Compound 3 has the same structure as 2. The three vanadium atoms in complex 5 are linked by four bridging oxygen atoms from two tridentate thme(3)(-) ligands to form a V(3)O(4) chain in which V-O bonds alternate in length. The V-O(isopropoxo) bond in 6 is cis to V=O, and the V-O(acac) bond trans to the oxo group is relatively long. The V(2)O(2) rings of complex 1 and the mononuclear 1:2 complex can be considered to be the basic building block of the trinuclear complexes 4 and 5 and the tetranuclear complex 2, acting to extend the vanadium-oxide framework. (51)V and (1)H NMR spectroscopic studies of the solution state of complexes 1-6 revealed dramatic differences in structural and hydrolytic stability of these complexes. Compounds 1 and 3 only remained intact at low temperature in CDCl(3) solution, whereas the mononuclear compound 6 could remain at ambient temperature for approximately 10 h. Compound 4 only maintained its solid-state structure at low temperature in CDCl(3) solution, whereas compound 5 was significantly more stable. The structural integrity of oligomeric vanadium-oxygen frameworks increased significantly when the coordinating alkoxide group showed more resistance to exchange reactions than the methoxide group. The solid state and solution properties of this new group of complexes not only testify to the versatility of VO(2)(acac) as a vanadium(V) precursor but also raise questions relating to solution structure and properties of related vanadium complexes with insulin-mimetic properties and catalytic properties.