Dalton Transactions Dynamic Article Links Cite this: Dalton Trans., 2011, 40, 3198 www.rsc.org/dalton PAPER Non-oxo vanadium(IV) alkoxide chemistry: solid state structures, aggregation equilibria and thermochromic behaviour in solution† K´ atia C. M. Westrup, a Thaiane Greg´ orio, a Danilo Stinghen, a Dayane M. Reis, a Peter B. Hitchcock, b Ronny R. Ribeiro, a Andersson Barison, a Davi F. Back, c Eduardo L. de S´ a, a Giovana G. Nunes a and Ja´ ısa F. Soares* a Received 6th November 2010, Accepted 5th January 2011 DOI: 10.1039/c0dt01547d The reversible thermochromic behaviour of homoleptic [{V(OR) 4 } n ] complexes in solution [R = Pr i (product I), Bu s (B s ), Nep (N) and Cy (C)] is accounted for the existence of an aggregation equilibrium involving dimeric and monomeric species in which vanadium(IV) is respectively five- and four-coordinate. Bulky R groups such as Bu t and Pe t (tert-pentoxide) prevent aggregation and therefore give rise to exclusively mononuclear compounds (B t and P t , respectively) that are not thermochromic. The complexes and their temperature-dependent interconversion were characterised by single crystal X-ray diffractometry, magnetic susceptibility measurements and electronic, FTIR and EPR spectroscopies in a wide temperature range. Equilibrium constants and enthalpy and entropy changes for the dimerization reactions have been determined and compared with literature data. Introduction Over the last 20 years, numerous solid state structures of vanadium-containing, homo- and heterometal alkoxides have been described. 1 Their large variety of structural motifs is deter- mined by the relative charges and sizes of the metal ion(s) and the alkyl groups, together with the denticity and the versatile binding mode of the alkoxide ligands. However, this wealth of crystallographic information has not been frequently accompa- nied by detailed studies of solution or gas phase behaviour of the complexes, despite the relevance of such information for predicting the dynamics of homogeneous catalytic systems, sol–gel/vapour deposition media and biologically relevant molecules in solution. Although variable-temperature 1 H and 51 V NMR studies have been instrumental in providing data on the fluxionality and nuclearity of vanadium(V) alkoxides in solution, 2 gathering similar information on lower oxidation state vanadium species has been hampered by the paramagnetic nature and the oxygen/moisture a Departamento de Qu´ ımica, Universidade Federal do Paran´ a, Centro Polit´ ecnico, 81530-900, Curitiba-PR, Brazil. E-mail: jaisa.soares@ufpr.br; Fax: +55 41 33613186; Tel: +55 41 33613180 b Department of Chemistry, University of Sussex, Brighton, BN1 9QJ, UK c Departamento de Qu´ ımica, Universidade Federal de Santa Maria, Camobi, 97105-900, Santa Maria-RS, Brazil †Electronic supplementary information (ESI) available: Experimental details, photographs of colour changes in complexes C and B s , electronic spectra of N and C, table of correlation times and rotational radii for mononuclear complexes, variable temperature and 77 K EPR spectra of C, B s and P t in solution and solid state, Van’t Hoff’s plots for I, N and C. CCDC reference numbers: 262786 (complex I), 705491 (N) and 794585 (C). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c0dt01547d sensitivity of the compounds. This poses interesting questions not only on the correspondence between solid state and solution structures, but also on the actual composition of a number of reaction mixtures. In the early 1960’s, based on careful synthetic work and extensive spectroscopic data, D. C. Bradley and M. L. Mehta proposed electronic and molecular structures for 20 homoleptic non-oxo vanadium(IV) alkoxides, correlating their results with those for titanium(IV) analogues. 3 Physical properties of the complexes of a given metal were then shown to depend on whether the alkoxide contained primary, secondary or tertiary alkyl groups. The solubility of the products in several organic solvents enabled the authors to determine molecular weights ebullioscopically. No clear evidence of change in nuclearity with varying concentration was found in those essays, but, based on polymerization degrees higher than 1 obtained for some of the primary and secondary alkoxides, it was suggested that the com- plexes with small OR groups could exist in solution as mixtures of monomers and dimers (or trimers), similarly to their titanium analogues. 4 Magnetic susceptibility determinations and electronic, IR and EPR spectra then recorded for the ethoxide and methoxide compounds, both in solid state and solution, supported the proposition of a dimeric and a trimeric nature, respectively, for the products. 4 However, no crystal structure determination was reported and no similar spectroscopic studies were carried out for the remaining complexes, apart from for the mononuclear [V(OBu t ) 4 ]. 4,5 Over the years, no significant progress has been made on the structural, electronic and/or magnetic characterisation of these homoleptic alkoxides, in spite of their catalytic and synthetic usefulness. 6 3198 | Dalton Trans., 2011, 40, 3198–3210 This journal is © The Royal Society of Chemistry 2011 Downloaded by Universidade Federal do Parana on 27 March 2011 Published on 23 February 2011 on http://pubs.rsc.org | doi:10.1039/C0DT01547D View Online