[4-tBu-2,6-{P(O)(OiPr)₂}₂C₆H₂Sn(PPh₃)Cr(CO)₅]ClO₄ – a salt containing a cationic triphenylphosphane-stabilized organostannylene transition metal complex

The soft phosphane donor moiety has been used in organotin chemistry (Izod, 2012; Hupf et al., 2014). Yet, ionogenic compounds containing tin cations and phosphane Lewis bases are scarce (Barney et al., 1999; Objartel et al., 2008; MacDonald et al., 2011, 2012; Weicker et al., 2013; Burt et al., 2014). Recently, we reported the synthesis of and density functional theory (DFT) calculations on the heteroleptic organostannylene chromium pentacarbonyl complexes [4-tBu-2,6-{P(O)(OiPr)₂}₂C₆H₂(X)SnCr(CO)₅] (X=ClO₄, OTf), and [4-tBu-2,6-{P(O)(OiPr)₂}₂C₆H₂(L)SnCr(CO)₅][ClO₄] (L=pMe₂NC₅H₄N (DMAP, 4-dimethylaminopyridine), Ph₃PO). Within the scope of these investigations, we also learned about the ability of the soft Lewis base triphenylphosphane, PPh₃ to stabilize such complexes but, at that time, failed isolating the corresponding representative (Wagner et al., 2013). Herein, we report the isolation and complete characterization of the triphenylphosphane-stabilized organostannylene chromium pentacarbonyl complex 2 (L=PPh₃, Scheme 1). Compound 2 might formally be rationalized as a push-pull complex (Thimer et al., 2009; Al-Rafia et al., 2011; Ghadwal et al., 2011; Swarnakar et al., 2014; El Ezzi et al., 2015). However, as it was demonstrated previously (Wagner et al., 2013), both the Sn-Cr and Sn-P bonds are highly covalent.

Scheme 1:

Synthesis of the ionic compound 2 containing a triphenylphosphane-stabilized cationic organotin chromium pentacarbonyl complex.

Compound 2 was obtained as a colorless crystalline material from a solution of the corresponding perchlorate salt 1 in acetonitrile to which an equimolar amount of PPh₃ had been added. Compound 2 shows good solubility in common polar organic solvents such as acetonitrile and dichloromethane.

The molecular structure of 2, as well as selected interatomic distances and angles, is shown in Figure 1.

Figure 1:

Molecular structure (SHELXTL) of 2 showing 30% probability displacement ellipsoids.

The hydrogen atoms are omitted for clarity. Selected interatomic distances (Å) and angles (°): Sn(1)-C(1) 2.176(3), Sn(1)-Cr(1) 2.5896(5), Sn(1)-P(3) 2.6665(7), Sn(1)-O(1) 2.3081(19), Sn(1)-O(2) 2.3471(19), Cr(1)-Sn(1)-C(1) 138.00(7), Cr(1)-Sn(1)-P(3) 119.30(2), C(1)-Sn(1)-P(3) 102.65(7), O(1)-Sn(1)-O(2) 152.18(7).

The Sn(1) atom is pentacoordinated and exhibits a trigonal-bipyramidal environment, where O(1) and O(2) occupy the axial and C(1), Cr(1), and P(3) the equatorial positions, respectively. The overall geometry is similar to that of the corresponding DMAP-stabilized derivative (Wagner et al., 2013). The Cr(1)-Sn(1)-C(1) angle of 138.00(7)° is smaller than in compound 1 (146.31(10)°) (Wagner et al., 2013). The Sn(1)-P(3) distance of 2.6665(7) Å in 2 is similar as the ones observed in Sn(SCH₂CH₂)₂PPh [2.614(5) Å] (Baumeister et al., 1986), [PhB(CH₂PPh₂)₃][SnCl] [2.6746(14) Å] (Barney et al., 1999) and [(Ph₂Ppic)Sn(Cl)][SnCl₃] (pic=2-picolyl) [2.6962(5) Å] (Objartel et al., 2008). A shorter Sn-P distance was observed in [SnCl₂(PMe₃)₂][AlCl₄]₂ with 2.5390(6) Å (MacDonald et al., 2012). Similar and longer distances were also found for phosphane stannylene Lewis pairs [2.6362(6)–2.7489(4) Å] (Freitag et al., 2013a,b) and for [PhP(CH₂CH₂S)₂ Sn(C₅H₅N)Cr(CO)₅] [2.756(1) Å] (Jurkschat et al., 1988). For [(oC₆H₄CH₂PPh₂)₂SnW(CO)₅], the Sn-P distances of 2.831(2) and 3.012(2) Å are much longer, however (Abicht et al., 1987).

During storage of compound 2 in CD₃CN solution in a sealed NMR tube, yellow crystals separated. These were identified by single crystal X-ray diffraction as trans-[Cr(CO)₄(PPh₃)₂], 3, which crystallized as a polymorph to known structures with similar interatomic distances and angles (Bao et al., 2001; Bennett et al., 2004). Compound 3 crystallized in the space group P1̅ with two molecules in the unit cell. As for structures of trans-[Cr(CO)₄(PPh₃)₂] published before, a higher symmetry than P1̅ is missing for all structures. The molecular structure of compound 3 is shown in Figure 2. Selected interatomic distances and angles are given in the figure caption.

Figure 2:

Molecular structure (SHELXTL) of trans-[Cr(CO)₄(PPh₃)₂], 3, showing 30% probability displacement ellipsoids.

The hydrogen atoms are omitted for clarity. Selected interatomic distances (Å) and angles (°): P(1)-Cr(1) 2.3298(10), P(2)-Cr(1) 2.3370(10), Cr(1)-C(41) 1.902(4), Cr(1)-C(44) 1.876(4), P(1)-Cr(1)-P(2) 177.65(4), C(42)-Cr(1)-C(43) 172.15(15).

A ³¹P NMR spectrum of compound 3 in C₆D₆ showed a signal at δ 75.3 ppm being identical with that reported previously (Beck et al., 1994).

A ³¹P NMR spectrum of the CD₃CN solution from which the crystals of complex 3 had been separated showed two major resonances at δ 37.3 [J(³¹P-^117/119Sn)=125/130 Hz] and 30.3 ppm [J(³¹P-^117/119Sn)=180 Hz], respectively, in an almost 1:1 ratio. The first resonance is assigned to the organotin(II) perchlorate RSnOClO₃, 4 [R=4-tBu-2,6-{P(O)(OiPr)₂}₂C₆H₂]. The chemical shift and magnitude of the coupling constant are close to the values reported for RSnCl [δ 37.8 ppm, J(³¹P-^117/119Sn)=113/119 Hz] (Henn et al., 2011). The second resonance is assigned to compound 1 (Wagner et al., 2013). A ¹H NMR spectrum showed two sets of signals for the aromatic protons of the ligand that are assigned to 1 and 4, respectively. A ¹H ¹³C HMBC spectrum showed cross peaks to the C(1) carbon atoms at δ 170.4, 1, and δ 182.4 ppm, 4. The low field shift of the C(1) signal, which is assigned to compound 4, showed the absence of the transition metal fragment. A similar trend was observed for RSnCl [R=4-tBu-2,6-{P(O)(OiPr)₂}₂C₆H₂] (δ 186.7 ppm) compared with RSn(Cl)Cr(CO)₅ (δ 171.7 ppm) (Henn et al., 2011).

The reaction according to Scheme 2 proceeds via nucleophilic attack of the kinetically labile triphenylphosphane at the chromium center (Scheme 2). The driving force for the formation of 3 is its low solubility in acetonitrile.

Scheme 2:

Reaction of compound 2 in the CD₃CN solution.