A Tungsten(I))diphosphorus, t Bu Analog and CuBr

Different varieties in the unbound unit, while seemingly trivial to the novice chemist, create variation in crystal formation. While the ^tBu substituted unbound and bound unit are analogous to formations of the unsubstituted species, only the isotactic, bromine containing polymer is successful in crystallization whereas the chlorine, bromine, and iodine polymers are obtained for the unsubstituted species. This phenomenon demonstrates the trend of increasing difficulty in crystallization as heavier substituted subunits are reacted.

The subunit is synthesized in a two-part, modified procedure. First, tungsten

hexacarbonyl is treated with ^tBu-cyclopentadiene while refluxing for 18 h in decalin forming [Mo(^tBu-C5H4)(CO)3]2.¹⁷⁹ Then, the mixture is charged with white phosphorus and refluxed for another 16 h. After the mixture is cooled and the solvent is evaporated the contents are purified on a column of SiO2.²⁷⁰ The polymer is formed by gently layering a cold (0 °C) 25/CH2Cl2 solution with a CuBr/CH3CN solution. The complex is crystallized by

concentration, layering with pentane, and cooling (4 °C) in darkness for 7 days (Scheme 8).

Attempts to synthesize the CuCl and CuI analogs using the same procedure proved to be inconclusive due to lack of single crystals produced. All substance produced was amorphous and immesurable by X-ray diffraction.

P P

The synthesis or characterization of complex 25, the basal subunit, has never before been reported. The solid-state projection (Figure 47) shows tungsten bound by two carbonyl groups, tert-butylcyclopentadienyl, and the electron rich diphosphorus in a tetrahedral W2P2

framework. The lone pair on the phosphorus atoms provides attachment sites for metals.

X-ray Data Collection for 25. A yellow, rod having approximate dimensions of 0.090 x 0.040 x 0.010 mm, was mounted on an Oxford Diffraction Goniometer Xcalibur

diffractometer. All intensity measurements were performed using the omega scans method (λ

= 0.71073 Å) with a graphite crystal incident beam monochrometer. Cell constants were obtained from a Full-matrix least-squares on F².

P W C O

Figure 49. Solid-state projection of C22H26O4P2W2 (25). Hydrogen atoms are removed for clarity. Selected bond lengths (Ǻ): P–P 2.0983(24), W–W 3.0171(6).

The cyclopentadienyl and carbonyl groups for the unbound and bound unit are consistently located in opposite directions. Each of the Cu₂X₂ rhombi for the polymer (26) are perfectly flat as shown from their angle summations (360°). On the other hand the Cu₂P₄ rings deviate (angular summation, 717.7°) from the ideal 720°. This deviation is more noticeable in Figure 50 which depicts the perpendicular faces of the Cu2X2 rings. This is further proven by the lack of linearity (164.5°) of the Cu···Cu···Cu component; the breach is most severe when one compares it to the other unsubstituted polymers. If these slight deviations are ignored, the polymer exhibits C2 symmetry. The P–P bond length is virtually unchanged when the unbound unit is compared to the polymer. The Cu–Br–Cu angle of the

tBu substituted polymer (26) is greatly reduced (25°) in comparison to the nonsubstituted analog (23) from the added steric bulk while the Cu–Br bond length remains constant.

X-ray Data Collection for 26. A red to orange, flat rod having approximate dimensions of 0.2869 x 0.0303 x 0.0120 mm, was mounted on an Oxford Diffraction Goniometer Xcalibur diffractometer. All intensity measurements were performed using the omega scans method (λ

= 1.54184 Å) with a graphite crystal incident beam monochrometer. Cell constants were obtained from a Full-matrix least-squares on F².

W Br

C O Cu P 1

1

1 1

1 1 1

1 2 2

Figure 48. Solid-state projection of C22H26BrCuO4P2W2 (26). Hydrogen atoms are removed for clarity. Selected bond lengths (Ǻ) and angles (°): P-P 2.0973(20), Cu–P2 2.2938(18), Cu–P1 2.3113(17), Cu–Br 2.4964(9), Cu–Br‟ 2.4560(9), P1–Cu–P2 105.98(6), P1–Cu–Br 111.24(5), P1–Cu–Br‟ 122.81(5), P2–Cu–Br 106.84(5), Cu–Br‟ 2.4560(9), P2–Cu–Br‟

106.99(5), Br–Cu–Br‟ 102.05(3), Cu–Br–Cu 77.96(3), Cu···Cu···Cu 164.50(3).

Figure 49. View down the crystallographic y-axis of complex 26.

Br Br Br

P Br P P

Cu P

Cu Cu Cu

Figure 50. Structure of 26 perpendicular to the faces of the Cu2Br2 rings. Tungsten atoms and their ancillary ligands are omitted for clarity.

One fortunate and expected corollary from the presence of the ^tBu group is increased solubility in common solvents such as THF, CH₂Cl₂, and CH₃CN while remaining insoluble in unsubstituted hydrocarbons like hexane or pentane. The greater differences and increased solubility allow NMR studies at both low temperature and room temperature. When

temperatures are lowered to –80 °C, the resonance in solution is slower than the radio frequency pulse and four specific peaks are formed that represent the four chemically inequivalent phosphorus atoms. Even at room temperature variances between the unbound unit and the polymer are noticed in the ¹H NMR; the signals for the cyclopentadienyl ring and the ^tBu groups are split from the polymer‟s lack of symmetry in comparison to the unbound

unit. One instance of splitting is also noted for the ^tBugroup in the ¹³C{¹H} NMR of the polymer in comparison to the unbound unit.

60 20 -20 -60 -100 -140 -180 -220 -260 -300 -340 -380 -420 -460

ppm

Figure 51. Variable temperature ³¹P{¹H} NMR spectrum of 26 in THF-d8/CH2Cl2 (3:1);

From bottom to top: 27, 0, –20, –80, –110 °C. –110 °C, δ = –180.95 (s), –218.14 (s), – 303.00 (s), –319.90 (s).

The ^tBu group substitution adds enormous stability in comparison to the unsubstituted cyclopentadienyl analogs. The positive ESI-MS shows no instance of the

{(^tBuCp)2W2(CO)4P2} basal unit being damaged under measurement, every peak reported is attributed to the formula Cu_NBr_N–1{(^tBuCp)₂W₂(CO)₄P₂}_X. This increased stability is also manifested by a higher melting point (220–222 °C) when compared to the unsubstituted bromine analog (120–122 °C). Meanwhile, the negative ESI-MS shows species that contain CO groups, and if this assignment is correct, it is not certain what the source is as the basal units are the only species that contain CO groups. The other major species are the expected CuN–1BrN units.

The IR spectra for the unbound species shows only three peaks attributed to the CO groups. The polymer shows 5 and even though these peaks are shouldered, they are

consistent with the unsubstituted chlorine and bromine analogs; the shouldering is even more intense for the iodine species causing the masking of one peak for a total count of 4.

Ultimately, this phenomenon is the result of disrupted symmetry experienced by all of these similar species.

2.3.4. Polymers and Dimer Synthesized from reactions of Tungsten(I))diphosphorus