Structural diversity of halocarbonyl molybdenum and tungsten PNP pincer complexes through ligand modifications

Abstract

This work presents a comparative study of a series of halocarbonyl Mo(ii) and W(ii) complexes of the types [M(PNP)(CO)3X]X and [M(PNP)(CO)2X2] (M = Mo, W; X = I, Br), featuring PNP pincer ligands based on a 2,6-diaminopyridine scaffold. The complexes were prepared and fully characterized. The syntheses of these complexes were accomplished by treatment of [M(PNP)(CO)3] with stoichiometric amounts of I2 and Br2, respectively. The modification of the 2,6-diaminopyridine scaffold by introducing NMe and NPh instead of NH spacers with concomitant modification of the phosphine moieties changed the steric and electronic properties of the PNP ligand significantly. While in the case of NH linkers exclusively cationic seven-coordinate complexes of the type [M(PNP)(CO)3X](+) were obtained with NMe and NPh spacers neutral seven-coordinate complexes of the type [M(PNP)(CO)2X2] were afforded. In the case of the latter, when the reaction is performed in the presence of CO also [M(PNP)(CO)3X](+) complexes are formed which slowly lose CO to give [M(PNP)(CO)2X2]. The halocarbonyl tungsten chemistry parallels that of molybdenum. The only exception is molybdenum in conjunction with the PNP(Me)-iPr ligand, where the coordinatively unsaturated complex [Mo(PNP(Me)-iPr)(CO)X2] is formed. DFT mechanistic studies reveal that the seven-coordinate complexes should be the thermodynamic as well as the kinetic products. Since [Mo(PNP(Me)-iPr)(CO)X2] is the observed product it suggests that the reaction follows an alternative path. Structures of representative complexes were determined by X-ray single crystal analyses.

Scheme 1. Oxidative addition of X₂ to fac-[ML₃(CO)₃]^z.

Scheme 2. Structural diversity of halocarbonyl Mo PNP pincer complexes.

Chart 1. PNP ligands used for this study.

Scheme 3. Synthesis of cationic seven-coordinate halocarbonyl molybdenum and tungsten complexes.

Scheme 4. Synthesis of neutral seven-coordinate halocarbonyl molybdenum and tungsten complexes.

Scheme 5. Synthesis of seven and six-coordinate iodocarbonyl molybdenum and tungsten complexes.

Fig. 1. Structural view of [W(PNP^Me-Ph)(CO)₂I₂] (5a) showing 50% thermal ellipsoids (hydrogen atoms omitted for clarity). Selected bond lengths (Å) and bond angles (°): W1–P1 2.4252(5), W1–P2, 2.4255(5), W1–N1 2.253(1), W1–C32 1.948(2), W1–C33 1.988(2), W1–I1 2.8924(3), W1–I2 2.8958(3), P1–W1–P2 113.69(1), I1–W1–I2 81.15(1), C32–W1–C33 73.46(6).

Fig. 2. Structural view of [W(PNP^Me-Ph)(CO)₂Br₂]·1.5CH₂Cl₂ (5b·1.5CH₂Cl₂) showing 50% thermal ellipsoids (hydrogen atoms and solvent omitted for clarity). Selected bond lengths (Å) and bond angles (°): W1–P1 2.4190(7), W1–P2 2.4203(8), W1–N1 2.240(2), W1–C32 1.953(2), W1–C33 1.984(2), W1–Br1 2.6871(3), W1–Br2 2.6444(3), P1–W1–P2 120.44(2), Br1–W1–Br2 79.43(1), C32–W1–C33 73.44(9).

Fig. 3. Structural view of [Mo(PNP^Ph-Et)(CO)₂I₂]·CDCl₃ (6a·CDCl₃) showing 50% thermal ellipsoids (hydrogen atoms and solvent omitted for clarity). Selected bond lengths (Å) and bond angles (°): Mo1–P1 2.4384(8), Mo1–P2 2.4385(7), Mo1–C26 2.002(2), Mo1–C27 1.922(2), Mo1–I1 2.9159(5), Mo1–I2 2.9131(5), P1–Mo1–P2 111.98(2), I1–Mo1–I2 82.89(1), C26–Mo1–C27 72.01(10).

Fig. 4. Structural view of [W(PNP^Ph-Et)(CO)₂I₂] (7a) showing 50% thermal ellipsoids (hydrogen atoms omitted for clarity). Selected bond lengths (Å) and bond angles (°): W1–P1 2.4237(8), W1–P2 2.4322(8), W1–C26 1.932(3), W1–C27 1.997(3), W1–I1 2.9013(3), W1–I2 2.8978(3), P1–W1–P2 112.93(3), I1–W1–I2 81.40(1), C26–W1–C27 72.45(12).

Fig. 5. Structural view of [W(PNP^Me-iPr)(CO)₂I₂] (8a) showing 50% thermal ellipsoids (hydrogen atoms omitted for clarity). Selected bond lengths (Å) and bond angles (°): W1–I1 2.9255(2), W1–I2 2.8999(2), W1–N1 2.2553(18), W1–P1 2.4616(7), W1–P2 2.4594(6), W1–C20 1.940(3), W1–C21 1.982(2), I1–W1–I2 79.855(5), P1–W1–P2 114.97(2), C20–W1–C21 69.54(10).

Scheme 6. Synthesis of cationic seven coordinate iodocarbonyl molybdenum and tungsten complexes.

Fig. 6. Structural view of [W(PNP^Me-iPr)(CO)₃I]I (10) showing 50% thermal ellipsoids (hydrogen atoms omitted for clarity). Selected bond lengths (Å) and bond angles (°): W1–I1 2.8835(14), W1–P1 2.5049(13), W1–P2 2.4945(17), W1–N1 2.244(3), W1–C20 1.991(3), W1–C21 1.982(4), W1–C22 2.021(3), P1–W1–P2 150.32(3).

Scheme 7. DFT calculated free energies (in kcal mol^–1) of bromocarbonyl complexes.

Fig. 7. Energy profile for the dissociation of CO from [Mo(PNP^Me-iPr)(CO)₃Br]Br (A) and formation of seven-coordinate [Mo(PNP^Me-iPr)(CO)₂Br₂] (E). The free energy values (kcal mol^–1) are referred to A.

Fig. 8. Energy profile for the formation of six-coordinate [Mo(PNP^Me-iPr)(CO)₂Br] (J). The free energy values (kcal mol^–1) are referred to [Mo(PNP^Me-iPr)(CO)₃Br]Br (A).