Bi- and tri-metallic Rh and Ir complexes containing click derived bis- and tris-(pyrazolyl-1,2,3-triazolyl) N-N′ donor ligands and their application as catalysts for the dihydroalkoxylation of alkynes

Abstract

A series of bi-topic and tri-topic pyrazolyl-1,2,3-triazolyl donor ligands (1a-d; 1a-c = 1,X-bis((4-((1H-pyrazol-1-yl)methyl)-1H-1,2,3-triazol-1-yl)methyl) benzene (X = 2, 3 and 4; o-C6H4(PyT)2, m-C 6H4(PyT)2 and p-C6H 4(PyT)2) and 1d = 1,3,5-tris((4-((1H-pyrazol-1-yl)methyl)- 1H-1,2,3-triazol-1-yl)methyl)benzene, 1,3,5-C6H3(PyT) 3) were conveniently synthesised in 'one pot' reactions using the Cu(i) catalysed 'click' reaction. Rh(i), Ir(i), Rh(iii) and Ir(iii) complexes with ligands 1a-d of the general formulae C6H6-n[(PyT) M(CO)2]n[BArF4]n (M = Rh, Ir; n = 2, 3; 2a-d; 3a-d) and C6H6-n[(PyT)MCp*Cl]n[BArF4] n (M = Rh, Ir; n = 2, 3; 4a-d; 5a-d) were synthesised and fully characterised. In solution each of the bi- or tri-metallic complexes 4a-d and 5a-d exists as a mixture of two (4a-c, 5a-c) or three (4d and 5d) diastereomers due to the presence of a chiral centre at each metal centre in these complexes. The solid state structures of complexes 2b-c and 4a were determined by single crystal X-ray crystallography and showed that each bidentate arm of these multitopic ligands coordinates to the Rh or Ir centre in a bidentate fashion via the pyrazolyl-N2 and 1,2,3-triazolyl N3′ donors. The intermetallic distances in these solid state structures vary from 8.66 Å to 15.17 Å. These complexes were assessed as catalysts for the dihydroalkoxylation of alkynes using the cyclisation of 2-(5-hydroxypent-1-ynyl)benzyl alcohol, S, to a mixture of two spiroketals, 2,3,4,5-tetrahydro-spirol[furan-2,3′- isochroman], P1, and 3′,4′,5′,6′-tetrahydro- spiro[isobenzofuran-1(3H),2′(2H)pyran], P2, as the model reaction. The Rh(i) complexes (2a-d), with the highest TOF of 2052 h-1 for complex 2d, were the most active catalysts when compared with the other complexes under investigation here. The Ir(i) complexes (3a-d) were moderately active as catalysts for the same transformation. No significant enhancement in catalytic reactivity was observed with the Rh(i) series bi- and trimetallic complexes (2a-d) when compared with their monometallic analogues. The bi- and trimetallic Ir(i) complexes (3a-d) were much more efficient as catalysts for this transformation than their monometallic analogues, suggesting some intermetallic cooperativity. Rh(iii), 4a-d, and Ir(iii), 5a-d, complexes were not active as catalysts for this transformation. This journal is © the Partner Organisations 2014.