J.P. Tassone et al. / Journal of Organometallic Chemistry 776 (2015) 153e156
155
assessing the
s
-donating properties of the parent phosphine [18].
introduction of the borate group in each case has an additive effect
1
The magnitude of the JPSe coupling constant is sensitive to the
steric and electronic nature of the substituents attached to the
phosphorus centre. In general, phosphine selenides of more
on the s-donor power of the phosphine. The greatest differences
were observed between 1′ and 5′ (
D
1JPSe ¼ 28 Hz), and between 3′
and 7′ (
D
1JPSe ¼ 29 Hz). In comparison, the 1JPSe coupling constants
1
strongly
s-donating phosphines display smaller JPSe coupling
of the selenides of Ph2PCy (726 Hz) and Ph2PtBu (717 Hz) are larger
[18e], suggesting the borate charges of the 4-substituted phenyl
substituent in 5, and the 4-substituted benzyl substituent of 7, have
constants than the corresponding selenides of weaker
phosphines.
s-donor
The 31P{1H} chemical shifts and 1JPSe coupling constants for the
selenides of ligands 1e8 (correspondingly identified as 1′e8′ in
Scheme 2) are given in Table 1. Unlike the parent phosphines
themselves, the selenides fall within a narrow range of chemical
shifts regardless of substituent or overall phosphine charge. In
contrast, the changes in the phosphoruseselenium coupling con-
a greater impact on enhancing the s-donor power of the phosphine
than some of the strongest electron-releasing alkyl groups. The
insertion of a second methylene spacer upon moving from ligand 7
1
to ligand 8 leads to an increase in the magnitude of the JPSe
coupling constant of the corresponding selenide (7′
/
8′,
D
1JPSe ¼ 10 Hz), and consequently to a decrease in
s-donor power.
1
stants among 1′e8′ are less uniform. The difference in the JPSe
In comparison, the insertion of an aryl spacer in 5′ to give ligand 6′
1
coupling constants measured for the selenides of the neutral li-
gands 1 and 2 (1′, 729 Hz [18f]; 2′, 730 Hz) is negligible. Compared
to 2′, the selenides of Ph2P(4-MeOC6H4) and Ph2P(4-tolyl) yield
leads to a substantial increase in the JPSe coupling constant
(5′ / 6′,
D
1JPSe ¼ 28 Hz). In fact, the negligible difference in
phosphoruseselenium couplings between 2′ and 6′ suggests that
the location of the tetraphenylborate group in 6′ (or perhaps even
1
lower JPSe coupling constants in the same solvent (722 Hz [18d]
and 726 Hz [18e], respectively), reflecting the electron-releasing
properties of the 4-methyl and 4-methoxy groups, and further
revealing the sensitivity of the 31Pe77Se coupling to even distant
modifications. Among the neutral phosphine selenides containing
its very presence) has no effect on the s-donor power of the
phosphine. Interestingly, the counterion of the anionic phosphines
appears to exert little influence on the phosphoruseselenium
coupling constant, despite the argument that the dipolar form
R3Pþ-Seꢁ of a phosphine selenide is the dominant contributor to
the phosphoruseselenium bond rather than the double-bonded
1
methylene spacers, the selenide 4′ yields a lower JPSe coupling
constant (725 Hz) compared to 1′ and 2′, which is consistent with
the substitution of an aryl substituent (i.e., Ph2PeC(sp2) linkage) for
an alkyl substituent (i.e., Ph2PeC(sp3) linkage) [19]. However, the
1JPSe coupling constant for the selenide 3′ is larger (735 Hz) [18f]
compared to 1′ and 2′. Interestingly, a recent kinetic study [18f]
on the oxidative addition of [SeCN]ꢁ to a series of tertiary phos-
phines revealed slower rates of selenide formation for Ph2PCH2Ph
1
canonical form R3P]Se [20]. For example, the JPSe coupling con-
stant measured for the selenide of the Bu4Nþ salt [3] of ligand 5
(700 Hz) is almost identical to that observed for the Ph4Pþ salt.
The trends in phosphoruseselenium coupling data observed
among the phosphine selenides were corroborated by the results
obtained from an IR spectroscopic analysis of the CO ligand vibra-
tions in the complexes cis-Mo(CO)4L2 and [Ph4P]2[cis-Mo(CO)4L2].
than for Ph3P, thus suggesting the former is a weaker
s-donor
phosphine as a result of the presence of the benzyl substituent,
which is consistent with the relatively larger JPSe coupling
For example, the A1 n(CO) vibration [21] observed in the IR spec-
1
trum of cis-Mo(CO)4(1)2 is higher than that observed for
constant.
[Ph4P]2[cis-Mo(CO)4(5)2] (2025 cmꢁ1 vs. 2015 cmꢁ1, respectively;
1
Much more notably, we observed that the JPSe coupling con-
Dn(CO) ¼ 10 cmꢁ1), consistent with the relatively stronger
s-donor
stants for the anionic phosphine selenides 5′e8′ are all lower than
those observed for their neutral counterparts 1′e4′, suggesting the
power of ligand 5 compared to 1. Furthermore, the IR data confirm
that the impact of the borate group decreases as it is positioned
Scheme 2. Synthesis of the selenides 10e80.