Laboratory Experiment pubs.acs.org/jchemeduc
Trispyrazolylborate Complexes: An Advanced Synthesis Experiment Using Paramagnetic NMR, Variable-Temperature NMR, and EPR Spectroscopies Timothy N. Abell, Robert M. McCarrick, Stacey Lowery Bretz, and David L. Tierney* Department of Chemistry and Biochemistry, Miami University, Oxford, Ohio 45056, United States S Supporting Information *
ABSTRACT: A structured inquiry experiment for inorganic synthesis has been developed to introduce undergraduate students to advanced spectroscopic techniques including paramagnetic nuclear magnetic resonance and electron paramagnetic resonance. Students synthesize multiple complexes with unknown first row transition metals and identify the unknown metals by correlating spectroscopic data to electronic structure. Students are assessed through an oral presentation of their spectral analyses and conclusions. Data are included in Supporting Information for institutions without access to an EPR.
KEYWORDS: Upper-Division Undergraduate, Inorganic Chemistry, Physical Chemistry, Laboratory Instruction, Inquiry-Based/Discovery Learning, Coordination Compounds, EPR/ESR Spectroscopy, IR Spectroscopy, NMR Spectroscopy, Transition Elements
■
We have created a structured inquiry experiment11 that introduces students to two forms of advanced spectroscopy (variable-temperature paramagnetic NMR and EPR) not typically taught in undergraduate laboratory courses, and rarely taught together.1 In this experiment, students are introduced to these techniques through prelabs using guiding questions. For example, guiding questions are used to introduce paramagnetic NMR theory by comparing and contrasting it to students’ prior knowledge about diamagnetic NMR theory. To introduce EPR spectroscopy, a short video was made that is accompanied by guiding questions. This video introduces students to the effect of a magnetic field upon electron spin and describes how this, along with hyperfine coupling, leads to spectral splittings, similar to those seen in NMR spectra. The guiding questions help students to focus on important aspects of the new material and build connections with their prior knowledge. In this experiment, students synthesize complexes using one common ligand, but different first row transition metals, consistent with the ACS Committee on Professional Training guidelines19 for experiments that require students to characterize related compounds. The learning outcomes for this experiment are to examine correlations between spectroscopy and electronic structure, to distinguish between diamagnetic and paramagnetic compounds using NMR, and to determine
INTRODUCTION
The use of spectroscopic techniques continues to advance inorganic chemistry, but few experiments have been developed that enable students to explore these techniques in a teaching laboratory.1−10 Experiments have been developed to introduce students to electron paramagnetic resonance (EPR) in order to determine the geometry of a complex3,6 or calculate its gvalue(s).1,4,5 Each of these experiments requires extensive reading or prior instruction on EPR theory. Nuclear magnetic resonance (NMR) spectroscopy has been used to demonstrate the effect of a paramagnetic metal on proton chemical shifts, through the comparison of Co(II) and Co(III) complexes.2 However, few experiments incorporate both paramagnetic NMR and EPR,1 and no experiments have been published that expect students to combine variable-temperature paramagnetic NMR and EPR spectroscopies for identification of unknown transition metal complexes. The degrees of freedom when students conduct laboratory experiments, known as inquiry, exist on a continuum that ranges from confirmation to authentic inquiry.11,12 The more choices there are requiring students to make a decision about experimental design and data analysis, the more an experiment resembles authentic inquiry. Inquiry experiments are intended to help students make connections between their prior knowledge and new information.12 However, only a few experiments developed for upper-division inorganic laboratory courses use inquiry elements.13−18 © XXXX American Chemical Society and Division of Chemical Education, Inc.
Received: May 5, 2017 Revised: September 8, 2017
A
DOI: 10.1021/acs.jchemed.7b00302 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
mixture, containing bis- and trispyrazolylborate (Figure 1). Students dissolve the ligands in a 50/50 mixture of dimethylformamide (DMF) and water, divide the ligand solution into two portions, and add one unknown metal salt to each, stirring until it fully dissolves. Students are told that one of the metals they have been assigned will produce a diamagnetic complex at room temperature, while the other metal will form an EPR-active complex. Students are told the four possible metals are Fe(II), Co(II), Cu(II), or Zn(II), but they are not told which two they have been assigned. After stirring for approximately 1 min, the solution contains a mixture of three metal complexes with coordination numbers of 4, 5, or 6 (the three possible combinations of the ligands, Figure 2). The mixture is extracted twice with 30 mL of
which metals are EPR-active. Students are also required to develop their communication skills 20 by making oral presentations in which they make claims about the identity of their unknown metal, on the basis of analysis of their spectroscopic data. This experiment was implemented in a hybrid organic/ inorganic advanced synthesis lab. The students completed organic synthesis experiments for the first 7 weeks. The present experiment was directly preceded by one where students were introduced to crystal field and ligand field theory through synthesis and UV−vis analysis of several cobalt complexes. It was designed to be carried out by students who have completed organic chemistry; completion of an inorganic chemistry lecture course is not a prerequisite. However, prior knowledge of electron configurations (particularly, those of transition metal ions), d-orbital splitting of octahedral complexes, high-spin and low-spin complexes, diamagnetic and paramagnetic complexes, and diamagnetic NMR spectroscopy theory is expected, and this material is reviewed in the prelab exercises.
■
EXPERIMENTAL PROCEDURE Students work in pairs, and the experimental procedure requires two 3 h lab periods over 2 weeks to complete. The ligands were synthesized for the students, prior to the start of the experiment, in a solvent-free melt reaction of a 3:1 molar ratio of pyrazole and a borohydride salt for 1.5 h (Scheme 1).21 This synthesis yields a mixture of bis- and trispyrazolylborate (Bp and Tp, Figure 1) compounds. A detailed procedure is provided in the Supporting Information. Scheme 1. Reaction between Pyrazole and a Borohydride Salt To Form Bis- and Rrispyrazolylborate
Figure 2. Three complexes that result from the reaction have coordination numbers of 4, 5, or 6.
toluene, and the solvent is removed via rotary evaporation. Students then prepare a column, using neutral silica as the stationary phase and toluene as the mobile phase, to separate the complexes chromatographically. The six-coordinate octahedral complex, typically the first to elute from the column, is collected and left to crystallize, open to the air, in a hood for 1 week. While some of the four-coordinate complexes have been well-characterized, complexes of the form MBp2 show very short shelf lives, even in the solid state (some as short as a few days).22 The five-coordinate (MBpTp) congeners have never been reported, owing to substantially shorter lifetimes, disproportionating rapidly to the two homoleptic complexes (MBp2 and MTp2). Consequently, we focused the spectroscopic component on the more stable MTp2 complexes.
Figure 1. Ligand mixture is made up of a bidentate (bispyrazolylborate) and tridentate (trispyrazolylborate) ligand (B in pink, C in gray, H in white, N in blue).
Week 2: Collection of Spectroscopic Data
The prelab for week 2 reviews general NMR theory. The prelab also provides a narrated video and a set of guided questions to introduce students to spin crossover (SCO) systems, EPR spectroscopy and its analysis, and paramagnetic NMR spectroscopy. This should be completed prior to arriving to the second week of this experiment. Each pair of students collected 4 IR spectra, 2 1H NMR spectra, and 2 EPR spectra. All IR spectra collected on a Perkin−Elmer Spectrum 100 FTIR by diffuse reflectance, using the neat compounds. Spectra were collected between 4000− 590 cm−1 and 690−590 cm−1 for each complex. A roomtemperature 1H NMR spectrum was collected for each complex on a Bruker Avance300 NMR spectrometer (νH = 300 MHz),
Week 1: Synthesis and Purification
The prelab for week 1 helps students to review key prior knowledge regarding electron configurations (including those of transition metal ions), d-orbital splitting of octahedral complexes, and the differences between diamagnetic and paramagnetic complexes and high-spin and low-spin complexes. Students should complete the prelab before arriving to conduct the experiment. This procedure can be performed under standard laboratory conditions; none of the materials are air sensitive. Each pair of students is assigned two unknowns (divalent first-row transition metal salts), and they are supplied with 6 g of the crude ligand B
DOI: 10.1021/acs.jchemed.7b00302 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
using standard conditions. All NMR samples (∼5 mM) were prepared in d8-toluene. The paramagnetically shifted spectrum of the Co(II) complex, and of the Fe(II) complex at high temperature, can be easily obtained by opening the spectral window to ∼250−300 ppm, and then by reducing the acquisition time to
