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46743-B3
Homoleptic and Heteroleptic Copper(I) Phenanthrolines: Sterically Enhanced Excited States

Kurstan L. H. Cunningham, St. Norbert College

 SEQ CHAPTER \h \r 1Background:

            This project focuses on nine unique phenanthroline derivatives designed to create sterically congested and electronically activated heteroleptic and homoleptic copper(I) complexes.  This modification should create longer-lived charge separation in the excited state as a model for creating copper(I) based solar energy conversion devices superior to ruthenium based devices.

Figure 1: (Left) The numbering sequence for the 1,10-phenanthroline ligand.

(Right) A typical structure for a bis-phenanthroline copper(I) complex with R groups

in the important 2 and 9 positions.

            Previous data from the grant proposal demonstrated that derivatives of the 3,4,7,8-tetramethyl-1,10-phenanthroline (tmp) enhance the luminescent intensity of the subsequent copper complex compared to the unmethylated derivatives.  Therefore, we dedicated the majority of our efforts this year towards the synthesis of alkyl derivatives of the tmp ligand using the procedure for the unsubstituted phenanthroline in Figure 2 below:

Figure 2: Reaction scheme for the nucleophilic aromatic substitution on 1,10-phenanthroline with excess lithium reagent.¹  Products recovered are both the mono and di-substituted compounds which are separated via column chromatography.

Results:

            The reactivity of the alkyl lithium reagents was significantly different using 3,4,7,8-tetramethyl-1,10-phenanthroline.  Following the reaction above, tmp underwent a unique, single substitution with CH₃Li creating 2,3,4,7,8-pentamethyl-1,10-phenanthroline (pmp) in a quantitative yield.  Using this anomaly, we efficiently synthesized the asymmetric 9-isopropyl-2,3,4,7,8-pentamethyl-1,10-phenanthroline (Figure 3) from pmp starting material.

Figure 3: Reaction scheme for the creation of asymmetric 9-isopropyl-2,3,4,7,8-pentamethyl-1,10-phenanthroline (ippmp)

            Surprisingly, neither CH₃Li nor C(CH₃)₃Li were found to react with the pmp starting material in the manner in which the CH(CH₃)₂Li reagent was found to do.  We hypothesized that the t-butyllithium was experiencing a steric hindrance towards the reaction site that was not encountered by the smaller isopropyl reagent.  Since same conclusion would not be feasible for the methyllithium; we initially believed the five methyl groups were deactivating the ring towards a nucleophilic attach.  This would account for the lack of reactivity of the “weakest” nucleophile of the series CH₃Li.²

            Investigating the t-butyllithium reactivity identified that only the mono-t-butyl derivative (70% yield) was collected with no evidence of a di-substituted product being created.  This supported our conclusion that the t-butyl group was too large to occupy both the 2 and 9 positions with methyl groups at the 3 and 8.  Once again, we used the high yield of the monosubstituted derivative to create asymmetric compounds as seen in Figure 4.

Figure 4: Reaction scheme used to create the 2-t-butyl-3,4,7,8-tetramethyl-1,10-phenanthroline (tbtmp).  This starting material was used to make both the 2-t-butyl-9-isopropyl-3,4,7,8-tetramethyl-1,10-phenanthroline (tbiptmp) and the 9-t-butyl-2,3,4,7,8-pentamethyl-1,10-phenanthroline (tbpmp).

            These two new monosubstituted and three new asymmetric ligands have been characterized by ¹H and ¹³C-NMR, GC-MS and melting point.  Pending data will be from submission for CHN analysis using a contract lab.  Future work will be to increase production of the asymmetric ligands for creating the homoleptic and heteroleptic copper(I) complexes.

            Additional future work will be the unanswered question of how to make the 2,3,4,7,8,9-hexamethyl-1,10-phenanthroline (hmp).  It is clear the neither tmp nor pmp will react with methyllithium alone to create the hmp.  If the methyllithium will not act as a nucleophile, perhaps it will act as a base (Figure 5).  Alternatively, another direction will be the use of N,N,N',N'-tetramethylethylenediamine (TMEDA) to increase the reactivity of the methyllithium to increase the likelihood of a nucleophilic attack at the remaining alpha-carbon.³

Figure 5: Reaction scheme as an attempt to create the  2,3,4,7,8,9-hexamethyl-1,10-phenanthroline.

            This project also involves the supervision of undergraduates⁴ as a part of the project as per the PRF mission for students  “...to become involved in advanced investigative research activities, in preparation for continued study in graduate school or employment.”⁵  This ongoing project will be the topic of both oral presentations and poster presentations from the current students during this academic year. 

References:

1) a) Dietrich-Buchecker, C.O.; Marnot, P.A.; Sauvage, J.P. Direct Synthesis of Disubstituted Aromatic Polyimine Chelates. Tetrahedron Lett., 1982, 23, 5291

2) March, J., Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 4^th ed., 1992, Wiley, New York, 1992, 252.

3) The Chemistry of Organolithium Reagents, Zvi Rappoport, Wiley, New York, 2004.

4) Project Undergraduates from grant proposal to current year:  Previous: Tim Berto (‘07), Ph.D. program at University of Michigan; Laura Kubista (‘08), Ph.D. program at SUNY at Stony Brook; Steve Kraft (‘08) Ph.D. program at Purdue University. Current: James Lindlof (‘09) and Ian Klein (‘10).

5)http://portal.acs.org/portal/acs/corg/content?_nfpb=true&_pageLabel=PP_TRANSITIONMAIN&node_id=1263&use_sec=false&sec_url_var=region1

 (PRF website accessed 09/03/2008 )

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