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«A Dissertation Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of ...»

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Note: We omit the fac-designation when discussing specific complexes. Also, to simplify the text, we shall use the terms, E, E', Z, or Z' isomer, to designate the isomers of the entire complex with the amidine ligand in the respective configurations.

4.2 Experimental Section Starting Materials. Re(CO)5Br was synthesized as described in the literature.24 Re2(CO)10, 5,5′-dimethyl-2,2′-bipyridine (5,5′-Me2bipy), isopropylamine, isobutylamine, tertbutylamine, benzylamine, anhydrous methylamine and ammonia in steel cylinders, and AgBF4 were obtained from Aldrich. [Re(CO)3(CH3CN)3]BF4 was synthesized by a slight modification of a known procedure.25 The synthesis of [Re(CO)3(5,5′-Me2bipy)(CH3CN)]BF4 (1) from [Re(CO)3(CH3CN)3]BF4 is described elsewhere.26 Elemental analyses were performed by Atlantic Microlabs, Atlanta, GA.

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spectrometer. Peak positions are relative to TMS or solvent residual peak, with TMS as reference. All NMR data were processed with TopSpin and Mestre-C software. For specific assignments of signals listed in the synthetic section below, please see tables in the text and Supporting Information.

X-ray Data Collection and Structure Determination. Intensity data were collected at low temperature on a Nonius Kappa CCD diffractometer fitted with an Oxford Cryostream cooler with graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation. Data reduction included absorption corrections by the multi-scan method, with HKL SCALEPACK.27 All X-ray structures were determined by direct methods and difference Fourier techniques and refined by full-matrix least squares by using SHELXL97.28 All non-hydrogen atoms were refined anisotropically. All H atoms were visible in difference maps, but were placed in idealized positions, except for N-H hydrogen atoms, which were refined individually where possible. A torsional parameter was refined for each methyl group. For compound 3, the contribution to the structure factors from disordered solvent was removed by using SQUEEZE,29 amounting to 4/3 molecules of acetonitrile per unit cell. For compounds 3 and 4, BF4– sites were shared by a small amount of bromide, and the occupancies of BF4– and Br– were constrained to sum to unity. The occupancies were fixed in final refinements for 3. The structure of compound 6 was determined from a crystal having more substantial substitutional disorder for the anion, apparently about 52% BF4– and 48% ReO4–. In compound 3, the isobutyl group of one of the two independent cations is disordered into two orientations with 0.725(7)/0.275(7) occupancy. Crystal data and details of refinements are listed in Table 4.1, except for compound 6, for which the cation is illustrated in Supporting Information.

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R = (∑||Fο| - |Fc||)/∑|Fο|; bwR2 = [∑[w(Fο2 - Fc2)2]/∑[w(Fο2)2]]1/2, in which w = 1/[σ2(Fο2) + (dP)2 + (eP)] and P = (Fο2 + 2Fc2)/3, d a = 0.0218, 0.0341, 0.0284, 0.0275, and 0.0331, and e = 1.2987, 0.9602, 0, 1.9993, and 2.9329 for complexes 2-5 and 7, respectively.

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Isopropylamine (52 µL, 0.60 mmol) was added to an acetonitrile solution (6 mL) of [Re(CO)3(5,5′-Me2bipy)(CH3CN)]BF4 (1, 0.04 g, 0.06 mmol), and the reaction mixture was stirred at room temperature for 24 h. The volume was reduced to ∼1 mL by rotary evaporation, and addition of diethyl ether (∼10 mL) produced a yellow crystalline material, which was collected on a filter and washed with diethyl ether and dried; yield, 40 mg (63%). An NMR spectrum recorded immediately upon dissolution of this material showed mostly one set of signals. 1H NMR signals (ppm) in acetonitrile-d3: 8.84 (s, 2H, H6/6′), 8.28 (d, 2H, H3/3′), 8.04 (d, 2H, H4/4′), 6.10 (b, 1H, NH), 4.51(1H, NH), 3.14 (m, 1H, CH), 2.47 (s, 6H, 5/5′-CH3), 2.07 (s, 3H, CCH3), 0.74 (d, 6H, 2CH3). However, an NMR spectrum recorded after 15 min shows an equilibrium mixture of E′ and Z isomer signals, as observed in the reactions monitored by NMR spectroscopy and described below.

Amine reactions of 1 (10 mM in acetonitrile-d3, 600 µL) were monitored by NMR spectroscopy; we refer to this as the 10 mM solution. The first spectrum recorded at 5 min showed only reactants. On addition of a 10% excess of isopropylamine (5 µL) to the 10 mM solution, NMR signals indicative of a mixture of E′ and Z isomers of 2 were observed within 30 min (at 6 h, E′:Z = 64:36), and these signals continued to grow (while maintaining the same ratio of isomers) until no starting complex remained the next day. 1H NMR signals (ppm) in acetonitrile-d3 (cf. Figure 4.2 for atom numbering): 8.84 (s, H6/6′), 8.74 (s, H6/6′), 8.28 (overlapping d, H3/3′), 8.04 (d, H4/4′), 6.10 (b, NH), 5.57 (b, NH), 5.33(NH), 4.51(NH), 3.69 (m, CH), 3.14 (m, CH), 2.47 (s, 5/5′-CH3), 2.07 (s, CCH3), 1.89 (s, CCH3), 1.21 (d, 2CH3), 0.74 (d, 2CH3). Slow evaporation of this acetonitrile solution yielded X-ray quality crystals of the E′ isomer. 1H NMR spectrum in acetonitrile-d3: identical to that of the bulk precipitate.

[Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NHCH2CH(CH3)2)]BF4 (3). The method described ′ above but with isobutylamine (60 µL, 0.60 mmol) produced a yellow crystalline material; yield, 35 mg (54%). 1H NMR signals (ppm) in acetonitrile-d3: 8.84 (s, 2H, H6/6′), 8.29 (d, 2H, H3/3′), 8.04 (d, 2H, H4/4′), 6.29 (b, 1H, NH), 4.52 (1H, NH), 2.49 (m, 2H, CH2), 2.47 (s, 6H, 5/5′-CH3), 2.08 (s, 3H, CCH3), 1.82 (m, 1H, CH), 0.56 (d, 6H, CH3).





On addition of a 10% excess of isobutylamine (6 µL) to the 10 mM solution, NMR signals of a mixture of E′ and Z isomers of 3 were observed within 10 min, and the reaction was complete the next day. 1H NMR signals (ppm) in acetonitrile-d3: 8.84 (s, H6/6′), 8.73 (s, H6/6′), 8.29 (overlapping d, H3/3′), 8.05 (d, H4/4′), 6.29 (b, NH), 5.85 (b, NH), 5.34 (NH), 4.52 (NH), 3.06 (m, CH2), 2.49 (m, CH2), 2.47 (s, 5/5′-CH3), 2.08 (s, CCH3), 1.87 (s, CCH3), 1.82 (m, CH), 1.22 (m, CH), 0.95 (d, CH3), 0.56 (d, CH3).

X-ray quality crystals of the E′ isomer of 3 were produced upon slow evaporation of the solution of the crystalline material (10 mg) in a 1:5 (v/v) mixture of acetonitrile/diethyl ether.

[Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NHC(CH3)3)]BF4 (4). The method described above ′ but with tert-butylamine (65 µL, 0.60 mmol) produced a yellow crystalline precipitate (yield, 32

mg, 49%), but the reaction time was longer (4 days). 1H NMR signals (ppm) in acetonitrile-d3:

8.85 (s, 2H, H6/6′), 8.30 (d, 2H, H3/3′), 8.06 (d, 2H, H4/4′), 6.11 (b, 1H, NH), 4.30 (1H, NH), 2.47 (s, 6H, 5/5′-CH3), 2.01 (s, 3H, CCH3), 0.80 (s, 9H, CH3).

On addition of a 10% excess of tert-butyl amine (6.5 µL) to the 10 mM solution, NMR signals of a mixture of E′ and Z isomers of 4 were observed only after 1 h (reaction time, ∼4 days). 1H NMR signals (ppm) in acetonitrile-d3: 8.85 (s, H6/6′), 8.73 (s, H6/6′), 8.30 (overlapping d, H3/3′), 8.06 (d, H4/4′), 6.11 (b, NH), 5.97 (b, NH), 5.38 (NH), 4.30 (NH), 2.47 (s, 5/5′-CH3), 2.01 (s, CCH3), 1.95 (s), 1.38 (s, CH3), 0.80 (s, CH3). The resulting solution yielded X-ray quality crystals upon slow evaporation.

[Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NHCH2C6H5)]BF4 (5). The method described ′ above but with benzylamine (66 µL, 0.60 mmol) produced a yellow crystalline precipitate; yield, 38 mg (55%). 1H NMR signals (ppm) in acetonitrile-d3: 8.64 (s, 2H, H6/6′), 7.96 (d, 2H, H3/3′), 8.05 (d, 2H, H4/4′), 7.15 (t, 1H), 7.06 (t, 2H), 6.91 (b, 1H, NH), 6.09 (d, 2H), 4.38 (1H, NH), 3.94 (m, 2H, CH2), 2.47 (s, 6H, 5/5′-CH3), 2.18 (s, 3H, CCH3).

On addition of a 10% excess of benzylamine (7 µL) to the 10 mM solution, NMR signals of a mixture of E′ and Z isomers of 5 were observed within 15 min, and the reaction was complete the next day. 1H NMR signals (ppm) in acetonitrile-d3: 8.76 (s, H6/6′), 8.64 (s, H6/6′), 8.27 (d, H4/4′), 8.05 (overlapping d, H3/3′ Z and H4/4′ E′), 7.96 (d, H4/4′), 7.38 (t, benzyl), 7.31 (t, benzyl), 7.22 (d, benzyl), 7.15 (t, benzyl), 7.06(t, benzyl), 6.91 (b, NH), 6.32 (b, NH), 6.09 (d, benzyl), 5.54 (NH), 4.45 (m, CH2), 4.30 (NH), 3.94 (m, CH2), 2.44 (s, 5/5′-CH3), 2.18 (s, CCH3), 1.84 (s, CH3). The resulting solution yielded X-ray quality crystals upon slow evaporation.

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described above but with methylamine (∼30 µL, volume approximate because the volatile methylamine was added from an inverted container) produced a yellow precipitate; yield, 32 mg (52%). 1H NMR signals (ppm) in acetonitrile-d3: 8.83 (s, 2H, H6/6′), 8.28 (d, 2H, H3/3′), 8.05 (d, 2H, H4/4′), 6.80 (b, 1H, NH), 4.50 (1H, NH), 2.25 (d, 3H, NCH3), 2.47 (s, 6H, 5/5′-CH3), 2.07 (s, 3H, CCH3). Anal. Calcd for C18H20BF4N4O3Re: C, 35.25; H, 3.29; N, 9.13. Found: C, 35.36;

H, 3.26; N, 9.02.

On addition of a 10% excess of methylamine (∼5 µL) to the 10 mM solution, NMR signals indicative of a mixture of E′ and Z isomers of 6 were observed within 20 min (E′:Z = 66:34) and continued to grow, maintaining the same ratio of isomers until no starting complex signal remained (∼6 h). 1H NMR signals (ppm) in acetonitrile-d3: 8.83 (s, H6/6′), 8.73 (s, H6/6′), 8.28 (overlapping d, H3/3′), 8.05 (d, H4/4′), 6.80 (b, NH), 5.90 (b, NH), 5.29 (NH), 4.50 (NH), 2.87 (d, NCH3), 2.25 (d, NCH3), 2.47 (s, 5/5′-CH3), 2.07 (s, CCH3), 1.86 (s, CCH3). Upon slow evaporation, the resulting solution yielded X-ray quality crystals of the E′ isomer.

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above but with ammonia bubbling through for ~5 min (medium flow rate) produced a yellow crystalline precipitate; yield, 25 mg (41%). 1H NMR spectrum in acetonitrile-d3: identical to that given below. Ammonia gas was bubbled through a 10 mM solution of [Re(CO)3(5,5′Me2bipy)(CH3CN)]BF4 in acetonitrile-d3 (600 µL), and the solution was monitored by NMR spectroscopy. NMR signals of a mixture of E′ and Z isomers of 7 were observed. 1H NMR signals (ppm) in acetonitrile-d3: 8.79 (s, H6/6′), 8.75 (s, H6/6′), 8.29 (overlapping d, H3/3′), 8.04 (d, H4/4′), 6.30 (b, NH), 5.93 (b, NH), 5.45 (NH), 5.33 (NH), 2.48 (s, 5/5′-CH3), 2.12 (s, CCH3), 1.83 (s, CCH3). When the experiment was repeated in CDCl3, a mixture of isomers formed. 1H NMR signals (ppm) in CDCl3: 8.75 (s, H6/6′), 8.60 (s, H6/6′), 8.27 (overlapping d, H3/3′), 7.92 (d, H4/4′), 6.15 (b, NH), 5.86 (b, NH), 5.58 (NH), 2.50 (s, 5/5′-CH3), 2.21 (s, CCH3), 2.17 (s, CCH3). Slow evaporation of this chloroform solution yielded X-ray quality crystals.

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(HNC(CH3)NHCH(CH3)2)]BF4 (2) in acetonitrile-d3 (600 µL) was treated with a fivefold excess of 4-dimethylaminopyridine (2.0 mg, 25 mM), and the solution was monitored by 1H NMR spectroscopy. A similar experiment was conducted in CDCl3.

4.3 Results and Discussion Synthesis. Syntheses of [Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NHR)]BF4 complexes were carried out in acetonitrile (R = isopropyl (2), isobutyl (3), tert-butyl (4), benzyl (5), and methyl (6)) at room temperature (Figure 4.2). For R = H (7), acetonitrile or chloroform was used.

Reactions were monitored at intervals of 10 min, 1 h, and 1 to 4 days (sometimes also 5 min, 30 min, or 6 h) by NMR spectroscopy. Times required for completion of reaction varied (∼6 h for 6;

∼1 day for 2, 3, 5, 6, and 7; and ∼4 days for 4). For compounds 2 to 6, the ratio of E′ to Z isomers remained the same throughout the course of the reaction. For 7, which has a symmetrical remote nitrogen group, the isomer designation is restricted to E and Z; experimentally, a trace amount of the E isomer was observed in addition to the major isomer, Z.

Structural Results. Complexes structurally characterized here, having the general formula, [Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NHR)]BF4 (R = alkyl, benzyl or H, Figures 4.3 and 4.4, and Figure C.1, Supporting Information), exhibit a distorted octahedral structure, with the three carbonyl ligands occupying one face. The remaining three coordination sites are occupied by the two nitrogen atoms of the 5,5′-Me2bipy ligand and a nitrogen atom of the neutral monodentate amidine ligand. Crystal data and details of the structural refinement for these complexes are summarized in Table 4.1. The atom numbering systems in the ORTEP figures are used to describe the solid-state data. The Re−C bond distances (not shown) involving the CO group trans to the amidine group are not significantly different from those of the other Re−C bonds.

Figure 4.3.

ORTEP plots of the cations in [Re(CO)3(5,5′-Me2bipy)(E′-HNC(CH3) NHCH(CH3)2)]BF4 (2), [Re(CO)3(5,5′-Me2bipy)(E′-HNC(CH3)NHCH2CH(CH3)2)]BF4 (3), [Re(CO)3(5,5′-Me2bipy)(E′-HNC(CH3)NHC(CH3)3)]BF4 (4), and [Re(CO)3(5,5′-Me2bipy)(E′HNC(CH3)NHCH2C6H5)]BF4 (5). Thermal ellipsoids are drawn with 50% probability.

Figure 4.4.

ORTEP plot of the cation in [Re(CO)3(5,5′-Me2bipy)(Z-HNC(CH3)NH2)]BF4 (7).

Thermal ellipsoids are drawn with 50% probability.

These [Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NHR)]BF4 complexes (R = isopropyl (2), isobutyl (3), tert-butyl (4), benzyl (5), and methyl (6)) have an amidine ligand in the E′ configuration in the solid state (Figures 4.3, and Figure C.1, Supporting Information), even though NMR data show that both E′ and Z isomers exist in acetonitrile (discussed below). The molecular structure of [Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NH2)]BF4 (7) reveals that this nonbulky amidine ligand has the Z configuration (Figure 4.4), in contrast to the structures found here for all other crystals.

The Re−N bond lengths of the amidine complexes (Table 4.2) are comparable to those found for typical Re sp2 nitrogen bond lengths, which range from 2.14 to 2.18 Å.30 Likewise, in all cases the bond distances from the amidine carbon (C16) to the two nitrogen atoms, N3 and N4 (see Figure 4.2), are closer to the average sp2 C double-bond length to N (C=N, ∼1.28 Å) than to the average sp3 C single-bond length to N (C–N, 1.47 Å).31 For example, in [Re(CO)3(5,5′Me2bipy)(HNC(CH3)NHCH(CH3)2)]BF4 (2) (Figure 4.3), the bond distances are 1.308(3) Å (N3–C16) and 1.339(3) Å (N4–C16). The N3–C16 (1.266(12) Å) and N4−C16 (1.359(12) Å) bond distances of 7 (Figure 4.4) are generally similar to the relevant distances (Table 4.2) of [Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NHR)]BF4 complexes 2 to 5.

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