Tris(1-pyrazolyl)borate (Scorpionate) Functionalized Polymers as Scaffolds for Metallopolymers Yang Qin, Chengzhong Cui, and Frieder Ja ¨kle* Department of Chemistry, Rutgers UniVersity Newark, 73 Warren Street, Newark, New Jersey 07102 ReceiVed February 11, 2008 ReVised Manuscript ReceiVed March 27, 2008 Since their introduction in 1966 tris(1-pyrazolyl)borates (Tp), also known as scorpionates, have been extensively used as polydentate ligands with widespread applications in catalysis and materials chemistry. 1 Their polytopic analogues with multiple Tp ligands attached to a suitable backbone have recently attracted interest as potential building blocks for the assembly of new metal-containing polymers. 2 For instance, ditopic ligands, which feature two Tp (or related) functionalities that are bridged by a phenylene 3 or ferrocenylene 4 moiety, were reported by Niedenzu and Wagner. They are promising as precursors to linear rigid-rod coordination polymers through metal-ligand coordination. However, such a stepwise polymerization process provides little control and hence often leads to low molecular weights (MW), broad MW distributions, and sometimes to limited solubility. An intriguing alternative approach is to incorporate Tp ligands into well-defined polymeric structures, which may then be used as scaffolds for new metallopolymers with novel optical, electronic, or magnetic properties. 5 Polymers with, for example, multiple phosphine or polypyridyl binding sites continue to play important roles as supported reagents, catalysts, and linkers in organic synthesis, 6 as luminescent materials, 7 and in the assembly of supramolecular polymers. 8 The selective incorporation of Tp functionalities into polymeric structures is expected to greatly expand the variety of polymer- supported ligands and ultimately that of metal-containing polymers; at the same time, it provides the versatile Tp ligands with unique polymer properties such as enhanced stability and favorable processability. We chose a polymer modification approach starting with poly(4-trimethylsilylstyrene). We have shown previously that 4-trimethylsilylstyrene can be polymerized by atom transfer radical polymerization (ATRP) to give functional polymers of narrow molecular weight and that excellent architectural control can be achieved. 9 The high selectivity of silicon-boron exchange reactions with BBr 3 then provides an excellent tool for attachment of the desired borate functionalities to polysty- rene. As discussed in our recent work, the alternative route of direct polymerization of the respective boron monomers can in some cases be advantageous for the preparation of copolymers, but generally polymerization control is more difficult to achieve. 9d Copolymers with different boron loadings were prepared by silicon/boron exchange to determine the optimal conditions for Tp functionalization. Poly(4-trimethylsilyl)styrene (M n ) 16.4 × 10 3 , PDI ) 1.11) was treated with 0.25, 0.50, 0.75, and 1.00 equiv of BBr 3 (relative to SiMe 3 groups) in CH 2 Cl 2 for 24 h (Scheme 1). Random replacement of the Me 3 Si moieties is expected to take place under these conditions, and integration of the 1 H NMR signal of the residual SiMe 3 groups relative to the other aliphatic and aromatic protons suggested boron loadings of 24%, 48%, 73%, and 97%, respectively (see Figure S2 in the Supporting Information). Thus, the degree of boron functionalization is very close to that expected on the basis of the feed ratio of BBr 3 . In-situ treatment of the resulting BBr 2 functionalized copolymer 1 with a slight excess of Me 3 - SiNMe 2 gave the B(NMe 2 ) 2 -modified random copolymer 2. An upfield shift of the 11 B NMR signal to δ ) 32 ppm (cf. 1, δ ) 56 ppm) is consistent with attachment of the NMe 2 groups to boron, as is the observation of a new broad peak in the 1 H NMR at δ ) 2.65 ppm. Integration of the signal for the B(NMe 2 ) 2 moieties relative to the SiMe 3 signal is in excellent agreement with the degree of boron functionalization expected on the basis of the feed ratio of BBr 3 and consistent with that determined for the BBr 2 -functionalized intermediate (see Figure S3 in the Supporting Information). The NMR data also confirm the high selectivity of these facile polymer modification procedures. The products were isolated by free-drying from benzene and their conversion to polymers 3-Na by treatment with pyrazole/ sodium pyrazolide in THF was studied. 10 Polymers 2 were dissolved in THF, then 2 equiv of pyrazole (HPz) and 1 equiv of NaPz were added, and the mixtures were kept at 80 °C under a gentle flow of N 2 for efficient removal of HNMe 2 . We found that the polymers with 24 and 48% loading remained soluble throughout the reaction period of 6 h. The polymer with 73% loading precipitated partially and very gradually, while the one with 97% functionalization precipitated almost immediately and almost quantitatively. 10 The THF-soluble polymers (24 and 48% functionalization) were directly studied by gel permeation chromatography (GPC) in DMF with 20 mM LiBr at 40 °C (RI detector), and the precipitates of the polymers with 74 and 97% loading were dissolved in DMF and also subjected to GPC analysis in DMF with 20 mM LiBr at 40 °C (RI detector). At 24% loading an average molecular weight of M n ) 23.9 × 10 3 (PDI of 1.21) is observed, at 48% M n is determined to 32.1 × 10 3 (PDI ) 1.31), and at 73% loading M n increases to 44.7 × 10 3 (PDI ) 1.37). This is consistent with the expected increase in molecular weight of the polymer with increasing content of Tp moieties, and an almost linear increase in molecular weight with degree of Tp functionalization is obtained (see Figure S1 in the Supporting Information). 11 Column interactions likely contribute to the slight band broadening relative to the profile of the silylated precursor in THF (M n ) 16.4 × 10 3 ; PDI ) 1.11); band broadening was also observed for the silylated precursor in DMF, in which it is only poorly soluble. Finally, the polymer with 97% functionalization (M n ) 40.9 × 10 3 ; PDI ) 1.48) shows a high molecular shoulder. This is likely due to poor control of the reaction with pyrazole/sodium pyrazolide due to almost immediate precipitation of the (partially substi- tuted) product. An aminoborane-functionalized polymer 2 with 30% loading was chosen for the preparative synthesis of 3-Na. The polymer 3-Na was isolated by removal of volatile materials and extensively washed with ether. A large upfield shift of the 11 B NMR signal to δ ) 1 ppm confirms conversion to tetracoor- dinate borate functionalities and is consistent with the chemical shift measured for the molecular analogue, sodium 4-tert- butylphenyltris(pyrazolyl)borate (see Supporting Information). The product is well soluble in THF and acetone, indicating smooth reaction without significant cross-linking. 12 To explore the ability of these novel Tp-functionalized polymers to bind to metal complexes, we chose to examine the formation of a complex of 3-Na (x ) 0.3) with the CpRu (Cp * Corresponding author. E-mail: fjaekle@rutgers.edu. 2972 Macromolecules 2008, 41, 2972-2974 10.1021/ma800310v CCC: $40.75  2008 American Chemical Society Published on Web 04/16/2008